Anyone dealt with puppy penis issues?

Our 16lb terrier mix has been with us for about 7 months and has been pretty well behaved all things considered. There is one behavior that baffles me and the vet.

Our boy will, for lack of a better way of explaining, hump his mouth until he his fully erect and then he struggles to get his red rocket back inside and it causes a lot of concern for myself and my partner. So far is hasn’t been stuck stuck but it feels like it is only a matter of time. Our vet hasn’t experienced this before and has actually chatted with other vets and they don’t know what’s going on either. It usually happens in the morning after his walk and breakfast. I’ve tried adjusting timings and whether or not we walk to no avail. We’ve tried calming probiotics and are planning on getting a calming diffuser to try as well.

This is an understandably very odd issue and it makes it difficult to leave him alone and even more difficult to leave him with friends and family. Any advice would be greatly appreciated.