I am having the same problem with my 2.5 yo DD. I wouldn't recommend the glycerin though. Number one, although a suppository seems like a simple deal, those tissues are delicate and the less they are tampered with the best. It is possible to cause small tears, trauma, and even perforation to the area when inserting suppositories. You may notice that rectal thermometers are rarely used anymore, that's why. Also, using medication when it's not needed, even over the counter, is really not a very good idea.
Second, it may be somewhat traumatic for you son to have that put in. Especially if he tries to fight you. So then he will be even more scared of the potty and associate the pooping with it. And later in life if he does need a suppository for something he will have bad associations already.
My DD does well with rewards, I would recommend a system like that. Or just wait a couple of months and try again. He may have matured somewhat and be more ready.
Good luck.