Man, I've been there too. What really helped me was to join the Y. It's the one indulgence weallow ourselves to pay for. It is invaluable in the winter when we would otherwise be stuck inside much of the time. I get to exercise, the kids get to play with other kids, and it uses up a considerable chunk of the day
If I'm really stressed out I take advantage of the full two hours that the kids can be in the child center. I exercise for an hour and then I read a book or newspaper. It took a long while to get to where it is this nice though. My youngest would not let me leave him in the center for a long time, so sometimes we went & I would end up spending an hour with him playing and we would leave--even that was better than being home all day though
I've also finally found some like minded mamas that I try to get together with 1x a week that's helped too!
All my kids went through "I only want mama" phases; especially my youngest. I started by going on small outings--returning library books, and slowly extended the time. He really needed reassurance that I would always come back. Like earthcore, my husband is the "on" parent when he gets home from work; he does baths, bedtime routine etc. What if you tried having him be the "on" parent with you there i.e. he odes the night routine with you in the room too, and slowly work to a point where you no longer are there. Perhaps that would work. Also, encourage dh to take your son out to do something fun. You'll get some time to relax, and dh and your son can bond doing something fun--even a trip to the park, some thing small.