Itchy red welts on belly.

Hi. I have a 4 month old 25lb American Pit Bull puppy name Vino. For the last past maybe three to four days he's been having a red like rash on his belly, almost looks like hives. He's been scratching as well and now has made red scratches on his belly and under his arms. He's all white so the marks are evident and the inflammation seems to be more visible at night. I've also noticed he's been genital licking much more lately (not sure if that has anything to do with anything).

Some background on him: He's 16 weeks old, APBT breed, we've had him since he was 9 weeks (6 lbs); we got him from a breeder who had his parents on site and after Vino there was one white puppy left from the litter. He still eats and plays normally thus far and we have him on Nutro Max for puppies (for about a month now) and he eats three times a day, tolerates it just fine. He's up to date on his shots and we've been taking him to our local clinic sponsored by the s.p.c.a. I do however need to take him to a "real" vet clinic because the s.p.c.a are being bullies about when we should neuter him, they tried to push us to neuter him when he was 3 months old; I said no (due to the research I conducted on my own) and they seemed to be pretty upset about it and wouldn't give him his third set of shots until we make an appointment for neutering (which I found pretty odd). Do plan on neutering him close to five to six months. We take him out (on walks) about three times a day with potty breaks here and there and he goes to the dog park to socialize (try to go at least twice a week (usually go a little more) but haven't been since the rash and itching. It's actually been about a week and a half before this inflammation occurred that he's been to the dog park (so I wouldn't necessarily blame the dog park solely).

For the itching situation we were instructed to give him an oatmeal bath (ground up oatmeal in a blender) and put it in his bath water and his shampoo is also an oatmeal formula. Also told to give him a tablet of Benadryl (little pink 25mg tab rolled in a little peanut butter) in 8 hours slots. We did that today, was that the right thing to do? Still too early to tell if it's working. Also what would you say is causing this skin problem? We really need help with this, we can't go to the spca clinic just yet because they simply do not take walk ins. We are in the process of searching and eventually taking him to a "real" clinic soon as well. Please help us out with this. Thanks so much!