How to know when ready to touch?

I've had my senegal Patrick 3 months now. He's about 8-9 months old.
We've been doing bits with him like step up and recall with moderate success. He takes food from our hands etc including small things such as sunflower seeds.
I want to try stroke him but am concerned it'll freak him out and/or I'll get bitten (not so worried about the bite). I'm more concerned it might be viewed as an attack or something.

So my question really is, 'is there a specific time I should wait for or should I just go for it?
Also what is the best way to approach him to pet him?

I don't have a senegal, so you might need to hear from owners, but for what it's worth....

My galah is a bit "beaky". I find she prefers it if I come at her neck from underneath/side, rather than trying to pat her on the head like a dog (although she doesn't mind that when she goes all gooey). I also always announce what I'm going to do and ask. "Can I give you a scritchy?" "tickle, tickle, tickle", "Who's a good girl, want a smoochy?" I think it helps if you are speaking calmly all the time if something is going to happen.

Try not to tense up if you do decide to give it a go. They can feel the tension. Relax, think good things and that all will be good, say it's all going to be good and you're going to try a pat, how about that?

Alice is the opposite! She'll let me put my hand underneath or in front of her face but if anyone else tries it they're probably going home with a bandaid on their finger... she prefers people to reach over the top of her head to scratch her crest which gives her the option of wriggling away if she's not in the mood.

I agree though, keep your behaviour calm, talk encouragingly and keep an eye on his body language. Bring relaxed is really important: A stressed person reaching out towards him is certain to spell danger, not 'hey, this person wants a cuddle!'. And if he doesn't like the way you approach you'll probably get a little lunge to say 'back off!', no big deal, just take your hand away and leave it for another time. I usually say 'sorry' if I try to touch Alice when she's not in the mood and get a lunge in response - seems only polite, parrots deserve to have their personal space respected just as much as humans do - but also I want her to know she can communicate to me not to do something without resorting to more agressive behaviour.

Zilla is an amazon, but she prefers me to scratch the top of her head/back of her neck and really doesn't like under the chin scratches.

I agree with being calm and slow and if she runs away or lunges she isn't ready for it yet, wait and work on it slowly.

If you do like the one finger scratching motion in slow motion, while saying scratch see how he reacts. If he backs away, that's a clear signal for you to back off. If he doesn't move, you can test his threshold little by little. But pay attention to his body language and respect his space. You have already gotten him used to your hands by hand feeding him treats, now try something different. But reward his good behavior. If you don't flood him, but just push him little by little you should be fine. Took me 3 months to get my Senegal to trust me enough to do a head scratch. Now she welcomes them.