He could have sustained trauma during the mating or if the two dogs were 'broken' apart. If it lasts longer than a day or two, I would take him to the vet.

I've had this happen shortly after a breeding, but it never lasted much longer than a few hours. I would consult you veterinarian to be safe. I hope he's not in pain! :(

If you intend on breeding him continually I would recommend After breeding you take a Clean, Wet, Soft cloth (something like an old Cotton T-shirt) And wrap around his penis for a couple minutes until the swelling goes down. This will help the swelling go down faster and keep him clean.

at around 9pm tonight i found my almost 2 year old yorkie, coco, lying on his back, feet up in the air, with his favorite jacket velcroed near his penis... (he was obviously playing with it) and his penis was swollen like a balloon! now he's not neutered (i was never really into the idea of getting him neutered..besides, he's never around any other dogs so i didn't think it would be a problem..) and this was the first time anything like this had happenned to him. i started panicking, looked online and found a couple of different ideas.. but after two hours, nothing seemed to help. it seemed as tho the swelling on his penis were sort of filled with semen ? (gross i know) so i decided to take coco, a stuffed animal, and lock them in the kitched (hoping maybe he just needed to finish up)...20 minutes later...and he's fine! now i'm not exactly sure if it was just a matter of time..or if he really did need to finish up and that stuffed animal helped.. but i just wanted to give you insight on what happenned with him and his swelling bc i know that i was in tears, afraid that something was terribly wrong, and would not want anyone else to worry like that! ps.. after tonight, i've decided to give the vet a call in the morning and sched. an appt to get him fixed! lol