Hi OP, Your DS doesn't have to visit a uni to apply to it, its good if he can, to get a feel for the place but it is absolutely not essential.
Your DS can apply for up to 5 uni's, on one UCAS application. Most people choose one aspirational uni (for if they do better than expected), 3 realistic ones and one that has lower entry requirements as insurance.
Once a uni has reviewed your sons application they decide whether to make him an offer or not. He can hold these offers until they all come in and then he can choose which one to make "Firm".
Once he has "firmed" an offer then it is final really, that's the one he is going to.
A uni might make him a conditional offer eg AAB or if they really like him they might offer unconditional which means they will still offer him a place regardless of his final grades.
If he doesn't get any offers he gets to choose another few unis and the process is repeated.
If he fluncks his A Levels and doesn't meet his offers there is always a chance to get in through "clearing", which is the system Uni's use to fill up any places they might have available once the exam results are known. Lots of people get into uni through clearing.
If your DS wants to do a course at Oxford/Cambridge or Medicine/Vet science then he has really left it too late for this year and would need to apply for 2020 instead.
My DS is taking a gap year after his A levels this year as he isn't completely sure what he wants to study yet.
Don't panic, its a difficult system if you haven't been through it before.
Also!! Don't forget to apply for student finance and to open a bank account for him (if he hasn't got one!)