No problem.
When he has met the conditions of his offer, it will track on UCAS, if it happens before a Friday, he should recieve a welcome email from the University the next day. If UCAS tracks on a Friday, usually he will get the welcome email the next worling day, which would be the following Monday.
When he gets the welcome email, it will contain his Student ID number and information on how to log on to his University portal. Once he logs on for the first time, it should be 24 hours from then that he will be able to apply for accommodation.
The accommodation is usually allocated and a room offered around 24-48 hours later. For some people they have recieved an offer on the same day they have applied, I reckon with the huge amount of applications around August time, it would be at least 24 hours after applying.
Its none of my business, however, there is a Santander bank on Campus and if your son opens up a student account, he will get a £1,500 overdraft(after £500 is in the account) and recieve a free rail card which gets him 1/3rd off rail travel all over the UK. If he is coming from England, this might be useful for trips home and would save some money, however, like I said, that's a personal matter and Im only mentioning it on the off chance that it may be some help.
If you need any more help, feel free to direct message me.
Calum