My son only wants to be a goalkeeper – 3v3 has thrown him a bit

I would keep his love of goalkeeping alive rather than treating it as something 3v3 needs to remove. You can explain very simply that in these games everybody gets a turn at stopping goals and everybody gets a turn at attacking. He is not being told he cannot be a goalkeeper; he is being given other football experiences as well. Then give him goalkeeper opportunities elsewhere in training. Shooting games, taking turns protecting a goal, handling activities where appropriate, or simply letting him go in goal when you play another small-sided game. What I would avoid is allowing him to spend every 3v3 standing on the line because he has decided that is “his position”. At six, loving goalkeeping is brilliant. He can keep that identity without missing all the other things the format is trying to give him.