How old's your little boy? My daughter's now 16 months, and I do a lot of going out for walks with her. On days when she wants to walk too I do a lot of marking time on the spot, as she understandably doesn't walk as fast as I'd like to! I also will walk as far as she'll want to walk, and then put her in the pushchair and go for a longer walk at my speed too. I've found it works quite well for keeping her happy, entertained and interested, as well as getting me off my butt!
I've also just recently got my bike out to start doing that too, but I'm not yet confident enough to put her on the back. I have picked up a child bike seat off freecycle though, so once I've got a helmet for her then I'm hoping to do that too.