I would absolutely get him an e-collar if he's still picking at the incision sites, or if they haven't yet healed...Is he on any after-surgery Antibiotics to prevent infection while his toe heals? This is obviously another reason to put a collar on him, to prevent infection...How is the site healing? Any redness, swelling, or discharge from the wound?
As far as his weight goes...Do you have a digital kitchen scale to weigh him on every day? I'm thinking that unfortunately you may need to start to take matters into your own hands here if you don't have a CAV or Avian Specialist near you who is willing to do anything except euthanize this poor little guy, who seems to want to live. At only 64 grams, this Quaker wants to live if he's still kickin' and has made it through surgery at that weight...That in and of itself is amazing to me...
So i'd start by getting him an e-collar, and making sure that you have a simple digital kitchen scale, and that you start weighing him on it every single day, at the same time each day (first thing in the morning before he eats his breakfast is best), and writing down the date, time, and weight each day in a notebook. Time to start tracking this yourself if they don't care...
Is he eating a good amount of his Harrison's pellets every day? How much do you actually give him? Does he finish the entire amount of pellets you give him each time, or is he usually leaving some? If he's eating a good amount of pellets every day (I suggest ALWAYS making sure there are pellets in his bowl all day long, every day for the foreseeable future, until his weight gets up to normal), then he obviously has some type of digestive or metabolic issue going on, because Harrison's is a very good pellet, and he should be gaining weight IF he's eating a normal amount of them every day, plus some fresh veggies and fruit...Have you ever fed him a seed-mix? The reason I'm asking is because in this situation, it's probably a good idea to add a healthy, low-fat seed-mix to his regular diet...Not to take away anything else that he's already eating every day, but simply adding a bowl of a seed-mix that contains no sunflower seeds, no nuts/peanuts, and no corn...There are several of these you can buy, just look for "Sunflower Seed Free"...While you want him to gain weight, you don't want him to develop Fatty Liver Disease in the process...
I'd also think about buying a baby bird hand-feeding formula, and not actually hand-feeding him it or even mixing it up, but sprinkling it on/into his pellets/seed-mix once a day not only to give him some added calories/protein, but more-so to give him some added Digestive-Enzymes and Probiotics, which are added to most-all baby-bird hand-feeding formulas. Many times when a bird is eating normally and they cannot gain weight, it's because their bodies are not manufacturing the proper amount of Digestive Enzymes, and this will make it impossible for his body to break-down the food that he eats, or to properly digest it, and then his body obviously cannot absorb much nutrition from the food at all...This would be easily diagnosed by doing blood-work, and the treatment for it is to simply add Digestive Enzymes and usually a daily Probiotic to his morning food...It won't hurt him even if it isn't his issue, but you'll be able to tell pretty quickly if it is if you're tracking his weight as mentioned every single day...
I would also highly recommend that you start giving him a Probiotic supplement once every single day...I don't know what you have access to in England, here we have several, one called "BeneBac", and another called "Qwiko Avian Probiotics", and we also have a ton of daily human Probiotic supplements available to us here as well, which can also be given to birds....There is actually a daily Probiotic supplement available here to us at any Walmart store and elsewhere, made by a company called Spring Valley, who is a major manufacturer of vitamin and dietary supplements for humans...They make a human supplement here called Spring Valley "Probiotic Multi-Enzyme Digestive Formula", which is a combination of both a strong Probiotic supplement AND a added natural Digestive Enzymes for people who have the same issue that your Quaker has,
including Lipase, Amylase, Pepsin, Cellulase, Pancreatin, etc. If you lived here in the US I would tell you to run to a Walmart and buy a bottle of this, a bottle of 200 tablets costs only $6 here...I have had to add this particular supplement to the diet of a Bearded Dragon and to the diet of a Love Bird here at the Rescue I work at, who had exactly the same issue your Quaker has, they wouldn't gain weight even though they ate normally...I would crush one tablet each morning and simply sprinkle the powder on the Love Bird's pellets and throughout his seed-mix (easier with the Bearded Dragon, I would just dust his live insects in it, lol)...This almost immediately turned them both around, and our Certified Vets did a blood-test to confirm that they both had an issue with their Pancreas in which it was not secreting proper amounts of Digestive Enzymes...Now, the treatment of buying actual prescription Digestive Enzymes is horribly expensive (many times dogs have this issue as well, and without adding Digestive Enzymes to their food every day they waste away to nothing). But simply adding this supplement worked wonders...So if you have anything like this available to you in England, or you can order them online, I'd highly suggest that as well...
You have to start trying something on your own, because this little guy isn't seemingly in pain, or unhappy, and he doesn't want to die or he would have already gone, so he's a fighter and he needs some help, and these Vets have given-up on him for whatever reason...So nothing wrong with "Going-Rogue" at this point with safe, natural supplements and dietary changes/enhancements...