I don't ski/snowboard, but my husband does and he is always amazed at how many little kids are on the slopes and at how good they are at it!
I think a lot of it has to do with his maturity and desire. If he likes the snow and has expressed to you that he wants to do it and does well listening to and following instructions he could begin skiing this coming winter. If he has never seen it, you may want to take him to watch and talk about him learning how to do it. Then get him in a lesson if the ski resort will offer one to children his age...if not you may need to instruct him yourself.
If you want him to have a good experience and want to continue skiing with you, please consider the obvious... proper clothing so he doesn't get cold and wet, proper size skis/boots and a helmet for safety and finally, stopping before he gets too tired and making a big deal about his progress so that he leaves the hill excited to do it again. It is so great to share something you enjoy with your children, which will make for many fun family memories and vacations. Don't force him though...he may be too little, not yet interested or scared to do it. You don't want him to not like it before he really tries it. Enjoy!!