I had a girl who was potty training since the last 6 months -off and on

She started off going potty here but then went to the same stage where she didn't like going here anymore. She would sit on my toilet but wouldn't go and would pee in her pullup. I would ask her why she wasn't going pee and she would answer "I only want to pee at mommy's house."

It passed though. She was pretty stuck in pullups and in that mindset for a couple of months and it got to the point where it just became natural for her to be in pullups and for me to change them everytime. About 2 months ago I started to think enough was enough and I talked to the mom about maybe switching back to trying again and this time no issue.

She did have a harder time remembering to tell me she had to pee because there were more friends and toys around and she did not want to leave and go pee and then miss the game or lose out on a coveted toy and have someone take it but she made great strides in a few weeks and is now pretty much 99% dry at my house and wearing undies during our waking ours.

I think it is nothing to really worry about. It is natural. Maybe he doesn't feel as comfortable at your house or pooing in front of others. There is a phenomenon of potty strangeness that I have read about whereas our bodies get too tense to poop in strange places. Like when you go on vacay somewhere and don't have a BM for a few days because somehow your own body doesn't feel comfortable or relaxed enough to do it on a strange toilet.

I am sure it will pass

I am not sure that you should buy a potty. They are messy to use and harder to sanitize and eventually he will have to get used to the big potty. Personally, I would hold on to my money and just wait for him to be more ready to try at your house and just keep putting him on so he gets comfortable and used to it