He thinks it is the perfect age to start. DS2 will be 4 in June. He has no critical judgement, naturally, being just 3 1/2, so I really think this sport is not the right choice for such a young boy. Cross country skiing is a different kettle of fish, and he is learning just fine.
Just because his 6 year old brother is doing slalom, it doesnt mean the 3 year old should too.
I think it is important to learn that you cannot do everything an older child does, while dh believes in treating them equally. Not possible, with such an age gap!