Does he not want to spend ANY time alone with you, with you? That would seem odd.
Other than that I think it's okay to want time where you are close by but not necessarily doing things together - I would find that stifling.
Do you live together? If not then this is even more odd IMO.
Agree it's nothing to do with only child or not.
Fundamentally it's a compatibility thing - if you want to be very close doing stuff together all the time and feel rejected when he wants to do other things, it might be that you'd get on better with someone who is more wanting to do things together rather than apart. That's not anything wrong with either of you, it just means you're not compatible in this area.
I think it would be odd if he's not paying you ANY attention when you're alone together and you don't live together, though.
But I must say DH can be like this, I don't mind it. Probably 70-80% of the time we're at home together without company, we're doing our own thing. I like it because I like doing my own thing as well. When we're out together or random times when we're at home we'll have a cuddle and a talk or sit and watch something together or whatever, but mostly we do or own thing and that suits us.
If this is a long term relationship then you need to be happy with the way that he is - he's not going to magically change into a different person. If you're not happy with the thought of living like this for the forseeable future, then don't continue. Life is too short to be with the wrong man!