Why is my baby crying?

My 6 month old baby boy is almost always fussing. Literally from the time we wake up to the time he goes to sleep. He will be fed and changed, and he doesn't stop if I play with him or cuddle with him. He has several activity toys and I will put him in/on each of them and he will be okay for 2 minutes at most, then start fussing. Now it's just a routine that I keep repeating but he is never satisfied enough to stop fussing for long. I will walk with him when I can, and he likes that, but I can't hold him/walk around 24/7, as I have another child as well. I'm wondering if he's just bored, but I don't know how he would be with all the attention he gets and all the toys and activity centers he has. He's not teething, and I have given him teething toys just in case, but nothing works. Can someone please tell me why he might be crying or give me more ideas of what I can do?