My daughters have covid, one is autistic and we haven't been able to stay away from either. We have done lft tests every day and a pcr test and we haven't caught it. They are almost out of their isolation period and We are very hopeful we won't get it as we have already passed the average time to show symptoms.
I am hopeful the vaccines have worked to protect us but there is one other factor here, neither of them got the cough which was happening on the bus when they likely picked it up. The 2 unvaccinated people in our home are also negative still.
There is nothing wrong with asking him to do lft tests at all, if he is going to a household where covid is present he is expected to do a pcr too 3 to 5 days after exposure. Test and trace would ask him to.
So ask him to do a lft before he comes home, ask him to do 1 every day for a week, you can grab plenty from the pharmacy.
If he develops a cough or other symptoms, isolate from each other as best you can.
He should not visit covid positive people but it doesn't sound like you can stop him.