He is not really old enough to eat seeds at this point all he is going to do is scatter them around. With babies I do offer millet spray because it's kind of a toy and food at this point in their development. If you decide you want to give him some seed as he gets older I would encourage you to buy the Volkman's brand specifically for eclectus. There are no dyes or fortification.
I am concerned that you have never hand fed and having your first be an eclectus as well!

There are many really BAD things that can go wrong with hand feeding as well as issues weaning. It is a huge pet peeve of mine when breeders hand over hand feeding babies to people with no experience, in the over 25 yrs that I have hand fed I have heard and seen too many horror stories to not always be worried when a situation like yours comes up. It has nothing to do with you or your abilities it comes down to hand feeding and correctly weaning is much more complicated than most people think.
At this point you absolutely should be putting other stuff fresh fruits and veggies cut small into his cage for him to pick at between hand feedings. I have typed it out more times than I care to count so if you read old eclectus posts here as well as the website "Land Of Vos" you will find all the food advise you could ever want. I do strongly encourage you to do the research on adult diet. The issues that come up with eclectus are almost always diet related.
Have you clipped him yet? If not that is another thing I would encourage you NOT to do yet. Baby parrots and eclectus in particular do MUCH better if they are allowed to fully fledge. What that means is that they learn to fly and land really, really well before you ever consider wing clipping. Even then eclectus are one of the parrots I do not think do well long term being clipped if you can avoid it.
When he starts getting fussy about eating don't force him to take the formula that is the way to aspiration and a dead baby. What temp are you feeding the formula at? Please tell me the breeder let you know to never use the microwave to heat it right?? When he refuses, cut back on the amount first and provide plenty of fresh foods for him to sample if he gets hungry and believe me he will. Eclectus for their size can pack away more fresh food than you can imagine once they get used to it. Next once you see him eating a good amount of the fresh foods you will cut out the morning feeding or the afternoon which ever he seems less interested in. Most I have fed give up the am feeding first because that is when I put in the most fresh then they give up the afternoon feeding and last they reluctantly give up the night feeding but it is super important that you don't force the process.
Keep us posted.. I do mean to scare the dickens out of you by saying that once something goes wrong with a baby they go down fast so it is imperative that you are diligent to what is normal and what isn't.
I also want to add that Red Sided Eclectus being a little larger than the smaller Solomon Island typically take longer than others to fully wean but to give you worst case I have a male Solomon that was 7 months before he gave up that final feeding. As I said to me I think forcing them to go off formula before they are really ready can have long term negative effects so I let them eat as long as they want it (mostly). So to me how ever long it takes, it takes I never go by what books or time frames say, I base it individually on each baby and what they show they are ready for.