Sometimes it can be just as simple as they know that someone can do something better than them eg. DD1 will come home and say "I can't run as fast as Harry but I can run faster than Jane" but sometimes it can be that they have been told by another kid that they aren't any good at something and they do take it on board. My DD1 is a pretty good drawer too and she often says she can't draw well - BUT this is because she gets frustrated as she knows what she wants her drawing to look like and the one she does isn't exactly as she wants it to be and she gets upset about it.

SO I think if you can work out why he is saying it then address it that way - it may not be because the others are saying he can't draw but a different reason.