Dogs no longer getting on

PinkChick

We have a 15 month old dog and my in laws have a three year old. They are the same breed. My mil minds our fella when we're in work so they've been together almost daily since ours was a couple of months old. They've always got on and been fine together. Played a lot, had their moments where they needed to be separated but on the whole were best buds. There've been a few changes in last while. I've been working from home full time for the last six weeks so our dog has been staying here. They still see each other but would be only once or twice a week and for much shorter periods. About four weeks ago our lad was neutered. He had an undescended testicle so it was medically advised. The older fella is intact. Recently they can't be together. The older fella keeps sniffing around our lads bits and he doesn't react well at all. He growls at him and bares his teeth. The sniffing is quite persistent and intense and it's also constant from the time they are in the same room but not sure why the reaction is so bad. I will be going back to work next year so they will need to be together again but at the minute, we are having to separate them completely. Any tips?

aonb

They are both males
you have taken your dog away from your MIL's dog on a daily basis
Your dog has been recently neutered
Your dog is reaching maturity
Lots going on...
Someone with more knowledge/experience will no doubt get back to you on this, but with all those changes, there is a bit going on with the two dogs, so dont worry for now. Its hard when you have a lovely set up with family dog sitting, so dont panic for now - some of the behaviour knowlegable folks will have some suggestions no doubt
What breed are they?

PinkChick

They are both shelties