I am just a parent but agree with Sendsummer. In the first instance he might look at the courses offered by Bath, Bristol and Durham and see how close they are to the courses he has taken or is currently taking. If there is a good overlap and he has good grades, this would provide a good basis for a requesting to transfer, and perhaps negotiation on what he might be given credit for. (Or do you like within commuting distance of a RG University?) Though probably unusual, I suspect he is not the first to want to move from London. Obviously he should look carefully at where he might live and whether the University would suit him.
(After an initial discussion to check procedures and course content you may be better writing so the request and arguments can be forwarded)
At the same time your DS might speak to/email his tutor at Imperial, explaining the problem and saying what he wants to do next and ask for support. Academics will often know or know of academics at other Universities and the level of their courses, so a good reference or active support will be very useful.
But he should first consider whether he wants to go back, or at least go back in September. Or is there any way he can complete this year by staying at home and commuting in for exams (a parent going with him?). I assume most of the teaching will be over, and that lecture notes can be forwarded to him via a fellow student of the University. Oddly, socially, third year in London can become easier with smaller numbers taking each option, then sitting together in the library and so getting to know each other better. The reverse may be true of somewhere like Warwick where friendship groups are formed in hall in the first year and then consolidated in shared houses, leaving a new late entrant isolated.
I really don't know, but if he wants time off could he complete his degree via the Open University, or failing that look at the online courses offered by the Big name American Universities. A mixture of both, ideally somehow passing this year's exams, might allow him to graduate without leaving home, and then if he wants to return, he could for a Masters. Some good Universities will offer either a one or two year Masters depending on the strength of the Undergraduate degree, so as long as he can demonstrate ability it might not matter that he has not taken one of the most taxing degrees, plus for many purposes a good maths degree is enough whether from the OU or elsewhere.