Hi, we're facing this with DS15 who starts her GCSE mocks on the first day of term. She's a self-starter so our plan is to expect her to revise a couple of hours a day (at least, I'd prefer 3, TBH) and to have a revision ban for Christmas Day and Boxing Day.
We have a DS6 so we will take him out for walks to leave DS15 in peace.
Also: early to bed, early to rise, a limit on Xmas chocolate (god knows how) but good, solid, nutritional food during the holidays. She will need to get outside for fresh air too, and will need some time off.
Relatives: we're on our own for most of xmas this year so she won't have too many distractions from family visits. You'll also need to limit time with friends unless it's a planned event.
I'd recommend a specific timetable so that you know when your DS is supposed to be working. Find out what type of learner he is, ie visual (so he draws posters of his revision topics) or whether he remembers best if listening to music. Some learn best if they teach other people, so you could try asking him about what he's been working on.
To be fair, your DS is 13 so he needs to do SOME work without over-stressing about it. Oh, WH Smith sell revision aids - there's an excellent one which I can't remember the name of, but it's essentially tabs of paper stuck on a ring so that you can jot down bits of info - call it a revision rolodex!
I hope this helps. Be firm that he has to do some work but don't get too het up about it. I've always bribed my DD so that she gets £5 for every A, £10 for each A and £2 for a B. After all, we go to work for money, so why not give them an actual financial incentive for doing well at school? It's helped my own DD do some work, I tell you!! She's now heading for 11 x A/A/B this summer (stealth boast) and we've raised the stakes so we could be heading for a second-mortgage come August, but she'll always have those GCSE results so it's worth it.