How old is your bird---Honestly, I wouldn't push it because if something does happen after your son decides to trust the bird, he will likely be freaked out for life. My cousin was bitten hard on the ear by the grey that grew up with us and it took her 20 + years to touch him again. She was about 5 and she still has a scar-- He was acting really sweet at the time and she expressed an interest in holding him, so my uncle (not knowing better) allowed her to do so---it may have been that she moved suddenly (as children often do, but it may have also been a bit of a display, as Neo was quite jealous of her).
Wait until your son is ready---9 or 10 is a better age, but even then, he has to want it (that way if he is bitten, it was his choice and no trust is violated). You could try taking his hand in yours (protecting it with yours) and allowing him to pet the bird, but I would wait a bit. Some kids are freaked out by unexpected flapping, so you might talk to him about why they do that. Until he has fatter fingers and better self-control, his fear probably is somewhat protective--if he were overly comfortable, he might end up doing something stupid without understanding the risk.
My Too often wants to stand on people's shoulders, but sometimes she will get on a power-trip about it if they don't have a bond---It's just like an attitude that she takes on... I can't really describe it- she definitely will feel people out and test them by doing the following 3 things: 1. trying to preen their eye (dangerous yes-- does she care? NO-if they let her, she is way cooler with them than if she us spurned)
2. jerking her head when they try to touch her while on their shoulder (as though to bite)
3. and often, running and pecking the back or top of their head (not biting- but A SOLID, deliberate thump)...
Based on how they react, she decides whether she likes them or not. If she doesn't, she will plan more tests for them in the future lol. Interestingly, she won't do it with people she knows REALLY well if they handle her often (e.g., my ex and me) and she won't do it with total strangers--- she only does this to familiar acquaintances with whom she has had limited physical contact...E.g., my mom, my sister, company that stays for more than a few days etc. She is almost ALWAYS nice the first time she is held by a new person, but after a few times (when the novelty wears off) the testing begins.
My point in telling you this is that just because the bird seems to want your son, does not mean that he will behave as he does with you on your son. Plus, imagine the damage if he did get hold of your sons face--as an adult, you can make the call for yourself, but if your boy is nervous, the bird will pick up on that.
Last story---Noodles is OBSESSED with my dad (even though we only see them like 2 x yearly).. Anyway, I can hold her when he is in the room (because she likes me a lot) but if she hears his voice and is on anyone else, she starts to get really agitated and will often bite whoever is holding her---not their face (although she did bit my mom's shoulder once) but an arm or hand if she feels that they are preventing her from getting to him---We aren't talking serious bites, but serious enough for those who have never been bitten by a cockatoo (bruising etc)! Her attitude completely changes if he enters the room (unless she is on me---and then she just goes into full-on begging mode--e.g., take me to him).
It's almost like in a movie when a spouse catches the other spouse in an affair and suddenly, the non-married entity is chopped-liver/pushed away (while the cheating party chases after their significant other saying, "Honey, it's not what you think!")
Noodles will act all sweet, but when she hears my dad, she is like, "Get your filthy shoulder out from under me you brute!"--I had no idea that she would do this to other people because she has never done it to me when my dad is around.
Moral of this ramble: You may think you know how your bird will behave, but you only have your experiences on which you can base that assumption---his may be different, as he doesn't have that bond.
