I just asked this a few weeks ago and got some awesome responses.
My daughter is 6 and I just started to have her really help around the house, load dishwasher, load/unload washer-dryer, sweep, put new garbage bag in, dust. I was really worried about my daughter hating chores, and resenting them and I was given the advise to make it fun, let her have her own cleaning supplies, let her pick them out at the store, work along side with her etc.. It's worked like a charm, I always ask her nicely if she wants to do something and she usually jumps at it, I don't force her to or demand her to and we don't have a chore chart or anything like that it it's very free flowing type of thing, if something needs to be done I'll give her a choice or she'll actually beg and plead "can I please vaccum" the vaccum is tricky she isn't quite strong enough to work it. She thinks scrubbing the toliet with the toliet brush is the coolest thing in the world go figure.
I'd say at age 3 start simple and work your way up. Maybe his job could be real simple like putting away the clean spoons from the dishwasher, after he master's that move him to both spoons and forks. Prehaps he could go get a garbage bag when you take the garbage out and you could work together to put a new bag in til he can do it himself. Just stuff like that. Buy him a duster and let him go crazy, at that age my daughter loved wet washcloths and would walk around the entire house washing things.
The hardest part for me was letting go and letting my daughter go at it, I always felt she wouldn't do a good enough job and I'd have to redo it anyways so why bother but to my surprise with practice I'm not really having to redo her cleaning jobs.
I also don't pay my daughter or anything. I don't feel I should pay her to help around the house she lives in. I think that get's into power struggle and then they are only doing chores for money and not because they want to contribute to their family and home. My daughter get's plenty of gratification by doing a good job and getting praised and seeing for herself how much a little help really helps. We can spend more time together. I'll actually make commments like wow since there is nothing else to clean what should WE do?
I did take her to toys r us this weekend and let her pick out a toy, which was a surprise. This was a unplanned outing I told her this was for all her help lately, good parent teacher confrence the other day and behaving and listening at both home and school. I don't plan to do this on a regular basis just occasionally when I feel she deserves it.