Sport will build self esteem and help your kid live with icthyosis, it helped me.
I played soccer very competitively and almost made the state team. 'Course I overheated like a cruiser without a radiator and my face looked like a beetroot, but sometime's I would (and still do) go til I'm about to pass out.
Just remember to keep the fluids and healthy foods up.

Sport also helped me make some good mates who accepted me for me and looked over the fact that I had different skin, it was a great experience overall.

But with the sport as well as the swimming I guess it depends on how serious the icthyosis is. Just remember to know when he's had enough and to inform the coach of it too.