Acclimatising a new cat to our one-cat household

[lemonsherbets78]

One of our neighbours sadly passed away from covid-19 last week. We have agreed (happily) to have his cat, he knows us, our cat and our home as he often comes into our house but our cat is just shy of 12 months and he is an older cat, not sure how old, and ours winds him up a bit. They get into scraps and scuffles. Any advice on how to integrate him into the home? I know nothing about having more than one cat, how to get him to know this is his home now not a few doors down etc.
What else do I do? Family aren't in a position to give me any details about him or his history. Our other cat is a rescue so the rescue did all the initial vet things too. Hopefully he's vaxxed and neutered but how would I go about finding this out without any info? Will need to wait until lockdown is lifted to get to vets so that may be an issue for another time