There are online opportunities but it will mean attending the college for the spoken language element and the exam days - so a minimum of 3. Sheffield College (Sheffcol) used to have a fabulous online facility. I don't know if they still do it but there are others who do.
Because he already has what's deemed as a pass at grade 4 you will have to pay for the 35 week course which I guess will be approx £600.
I did it. It was an amazing experience. The tutors are far more involved than one would expect. It becomes a real personal development experience. I thoroughly recommend it though I'm not sure it still runs due to funding cuts.
Alternatively, most FE colleges will run a night class from September with a similar cost. I have known students who already have a C(4) to need an upgrade to teach in Wales.
Other colleges may offer a fast-track programme for the June exam. This is more unusual because of funding but they are out there. Again, with a grade 4, it would have to be paid for.
It's generally not possible for one student to enrol at more than one college though so that might be a consideration.
Good luck to your son and well done for his achievement and ambition. There will always be time, a year or two down the line, to attain his personal target.
I went from a C to an A*. I had to work for it though!