Male Guinea Pig with trouble pooing

Question
Took him to the vet when I noticed he was pooing less frequent and I also noticed he was having trouble pushing them completely out.  Vat checked him over and said he was bright enough and still is eating and drinking.  Took him back to the vet next again day as he was still having trouble, the vet gave him an injection and some sort of lactose.  He is managing to poo whole pellets sometimes but sometimes not.  He curves his back alot when he is trying to pooh and puts his mouth down to the genital area, when I watched him from the side I seen something protruding from round about that area.  IUt was probably about an inch long, this being my first guinea pig could this have been his 'man thing' or do you think he maybe he has something blocked and it was that that was getting pushed out.  I was ready to try and grab it but then I decided not to in case this was his genitals, Is their thing about an inch long.  I am stuyck what to do next because as I say he is bright most of the time and he is still managing to get some pooh out but nothing like what he used to.  Any suggestions would be appreciated.

Answer
You don't say how old your Guinea Pig is. If it is a young Guinea Pig then the vet has probably given him a laxative to try and make him poo. Older Boars can sometimes get a compact anus with age and they need help in going to the toilet on a regular basis.

His penis is situated between his  back legs not at the back of his body. If you see anything other than poo from his anus it could be a prolapse and he must go back to the vet.

Other things that can stop guinea pigs from pooing are foreign bodies or growths