He needs to set some time limits on the computer. I work at home and when I have a big project, I spend a lot of time on the computer (can be 60 hours a week). When I don't have as much work, I sometimes still spend a lot of time on the computer because I am used to it, but I am normally just fooling around. My husband has had to remind that he would like to spend time with me, and that I should set a time for getting off, so I do. That really helps us.
Your husband should set specific work hours, and during those hours he works, plays a little if he has time, and then he gets off. If he has to work extra, he does that, but then goes back to his normal schedule when he doesn't need to work so much. My 3 year old son is home with me half of the day, and I used to never get work done when he was here. I had to actually spend time with him or get no work done and have an unhappy kid. I never resorted to the tv though, and still don't (other than a conference call last week I had to do), we do things together, and it is a nice break from work. Maybe if you told him in advance that you would like him to watch your son, and give him different ideas on what to do with him (park, legos, etc), and tell him it will be like a little break from work, he will try. Tell him that is important to you (really all 3 of you) that he spends quality time with your son. If he still doesn't get it, get a babysitter. You should also try to get the too much working/playing thing fixed, it can really ruin a relationship. At least talk to him and tell him how you feel, and do a date night like others recommended, to reconnect to each other.