Hi Pink, how you doing? tricky one this. At the mo, my wee guys not quite three, but his pal (couple of months younger) does ALL the things you say your son does. Pal A must copy everything that Charlie does, no matter if it's asking for a food he doesn't like (which he then refuses to eat) or wanting to play with the toy Charlie has. His mum has adopted the "like it or lump it, your decision is final" approach suggested here, because he was having major paddys if he didn't get his own way, but in the 2 months of trying it, nothing has really improved. Here's hoping....
On the positive side, my friends wee boy demonstrated this copying behaviour ALL the time, with all his classmates (he's 5). But, after a good few months of effort by his parents, nursery and then primary teachers, it finally seems to be abating and he's now more happy to stick with his own decisions and choices. His nursery teacher suggested it was partly a confidence thing, and that the wee guy didn't have faith in his own decisions, hence what everyone else chose suddenly seemed more appealing or a "better" choice. They tried lots of praise and approval for any/all choices he made, be it toys, drinks, colour of crayon etc and it really does seem to be working now. Think you have to just persevere with the "you've made your decision, end of story" plan babes, there's not much else you can do really, just try to reinforce to him how good his choices are at all times. Good luck, having seen it happen close at hand, I know how frustrating this problem can be. xx