There is a greenish tinge especially around his eyes in your brightened edition (i.e. the first picture shown), so that may well have come up with your brightening up a photo that is originally underexposed. But ok, I feel, underexposure may well happen when you do "firsts", and try to achieve them with ambient light only, I don't mind it too much, despite the fact that underexposure IS a technical flaw, of course. But I think you captured your son well here. His smile shows that he is half happy half wondering how it'll turn out what his dad is just doing, and I like that expression!
And he has lovely brown eyes. The greenish tint in your edited version is taking away from them, which is a pity.