Long post, beware...
Alexw I might be able to help a bit. I'm a dad of three (the youngest born in July) and I find it very hard to "get in the mood" lately. I'm finding myself wanting to stay up and watch TV and go on the net and generally have "alone time" a lot.
Cant help on the drinking thing because I've never touched alcohol myself, however, men do like time to themselves. Does he go out with mates a lot? I find it difficult to get time for this and only manage the odd footy match with my Dad twice every month. Cant even get over to watch it on the TV for "male bonding" sessions these days either.
I think the best advice I can give (but please don't blame me if it goes wrong) is to sit down alone with him and let him do the talking, in case there is something on his mind. I find my wife pushing me to talk leads to a lot of tears on my part, but it helps. As long as you don't say to much, or suggest too much either. Doing something special for him might help too, like combining the internet or tv to something sexy, IF the spark has gone in his wotsit.
In my POV though its generally beacuse we've had the same routine for too long. Pushing will get nowhere and one tip: If you say to him "Do you want to talk?" and he replies "no" rather sharpish then leave the house and let him stomp around a bit.
Going to the Doctor is a good step though. He might just need to talk to someone who you aren't connected to, or someone he is close enough to.
I hope I've been some use to you and, sorry again for the long post.