My daughter was born with very low muscle tone, and so her gross motor are behind (not REALLY behind, she's just three, and can walk, run, skip, but not jump, and she has trouble climbing, etc. on the playground without a LOT of practice) but her fine motor is great, so we were told by her pediatrition to really encourage her fine motor... she started using playdough scissors with playdough around age two (and seemed to hold them perfectly the first time) she now loves to practice cutting straight lines with her small preschool scissors. I have to add that my daughter is really careful, and understood the concept of 'we only cut PAPER' right away, so I don't worry about having them within her reach.
I would start with playdough if you can, with playdough scissors. That is really easy to snip and builds confidence. I guess you could use regular preschool scissors too. It's a tough action to get the hang of. Then try construction paper. Anything finer than that is harder...
It seems to be the opening of the scissors rather than the closing that some kids have trouble with, so look for the kind of scissors that spring back if you think that might be your sons problem. I havn't used them myself, but I know other families who do.