I remember the 'how on earth am I going to fill these hours' feeling, too. And it is horrible and I have so much sympathy for you but be assured it really will pass and soon he will be demanding you join in some complicated game that only he knows the rules of when you really want to cook the dinner instead. I'm sorry, it's really tough and really boring but it is only such a short time in terms of your whole life with him.
You know, it might be talking/communication, too. In the next few months he will probably get a word explosion and talking will relieve a lot of his frustration as much as being mobile would. In the mean time, you could also try learning Makaton with him - look for Something Special on CBeebies or get a book from the library. Actually, I know it's not recommended but I had a child who was immobile until 14 months and I used television (Teletubbies DVD and CBeebies) to alleviate her boredom. It did work and doesn't seem to have had any unwanted side effects. She's three now, can't be arsed with the television and talks just fine, in fact maybe a little too much. It would also give you a much needed break. I'm obv not suggesting you sit him in front of it for ages but when you are really stuck, maybe half an hour or twenty minutes of something you think is all right wouldn't do much harm and may help you feel less trapped!
Trying to think of things that amused my DD at this age. Hmmmm.
Clapping games - get him to hold his hands up and you do the complicated bits.
Singing.
Playdough (would highly recommend making your own)
Books - particularly anything with feely bits!
Building a tent from chairs and a blanket and both sitting in it and having a snack there. Watching you doing the construction will be as interesting as the rest of it - string it out!
Playgrounds were v popular, not least because she could watch the older children.
Umm, I can't think now! But honestly, this is pretty normal and you will get past it, I promise.
Also, I would also bf if not too painful for you. He's going through a lot of changes at the moment and it may help him feel more secure, if that's what he's used to.