My son had a very hard time potty training, with a lot of backsliding (and a lot of poop and pee cleaned off the walls!)
He did the very best when I didn't even mention it to him. If he had an accident, I'd have him help me clean it up to the degree he was able (and go get his own change of pants, etc.) but didn't make a fuss about it or yell. It was like he was so worried about my reaction that he tensed up and nothing could come out.
In terms of the constipation, you may want to talk to your pediatrician about getting a child-safe laxative, if you haven't tried one already. Little Remedies makes an over-the-counter one too. And there are suppositories you can use if you get desparate (again, check with your pediatrician) so that you don't have to go digging again.
But believe it or not, my very best luck was with prunes. I treat them like candy (i.e., they can only have one or two), feed them whole ones (you can buy them in the baking aisle, like raisins or craisins) and it really works with getting rid of constipation - a couple of prunes even works for me!
If your son is close to his half-birthday he may be backsliding because of development in another area. My ECFE parent educator is always telling us about how until about age 7, kids go through a massive development stage near their birthday and six months in-between (their half birthday). During this stage, their brains & bodies are working so hard on the new skills that some of the old skills get temporarily "forgotten."