I shouldn't worry. I was engaged at 15 then again at 16 and then at 18. I married at 23. None of the boys I was engaged to is my DH. I am not sure why you mentioned her nationality. Is she in Sri Lanka or UK?
I know that your son is an adult but I think that you are right to be concerned.
These days it is hard enough for a young person to find a job which pays enough to support themselves. If the young woman doesn't already have the right to live and work in this country, the way the immigration laws are framed, it is likely that your son would have to support them both with no recourse to public funds.
I know it isn't very romantic but your son would be wise to forget planning a wedding until he is sure of the financial commitment. Like any contract, marriage comes with rights and responsibilities and this marriage may mean that the young man would have to rely heavily (and perhaps unreasonably) on other members of his family.