It's difficult. I agree with you that kids' holidays should be for R+R but there's another side to the argument too.
Certainly at the end of Year 11, your child will be sitting a huge number of GCSE exams. It's entirely possible that he will be sitting exam-style GCSE modules at the end of Year 10 - mine (DD15 - Year 10) is in the middle of these right now. They are a percentage of her total GCSE mark.
If the first time your child had to prepare to revise, revise for and sit exams was for these important ones, he would possibly be very unprepared and out of his depth and may, in fact, not achieve his potential.
It is possibly a better idea that schools get children used to the idea of sitting exams every summer so that they have plenty of time to learn important lessons about how they work best so that by the time they are sitting public examinations, they've got their techniques sorted.
Clearly the results of these exams/ tests that your boys are taking this year don't REALLY matter, in the sense that they are not on their CVs forever but going through the processes may be very important for them.
So, I guess all the other long school holidays are for R+R and May half term is a time to balance out a bit of revision and preparation for the upcoming exams with a bit of relaxing and seeing friends.
My two are 15 and 17 now but have been having internal exams after May half term every year since Year 7.
What's usually happened in our house when my two were younger was this:
Usually spend the first weekend relaxing but also spend a bit of time getting their stuff sorted out ready to revise, ie: they tidy out their school bags and make sure they've got all of their notes/ books etc, we make a trip to Smiths to get any stationery they need (little cards, highlighter pens etc) and they make a revision time-table. For this age, say approximately four sessions a day of 45 minutes each for the rest of half term and make sure that each of the subjects that they do is given equal weighting.
It's then a case of getting on with it. Every day, our routine would be that they would get up and showered and breakfasted and have a bit of a doddle around but try to do three sessions (with breaks for a drink/ snack/ wee/ wander about) in the morning. Proper break for lunch then sit down and tackle the last session. Knock off, all done, at about three o'clock and then go out and do something nice - go for a walk, make a cake, see a friend, go to town/ cinema/ bowling/ a swim or just play on the games console or collapse in front of the telly - whatever they want to do. Then a reasonably early night so that they can get up and do it all again the next day.
While they are revising, it's a good idea to make a list of all of the topics for each subject and tackle them methodically. One of mine likes to just make her own notes on A4 paper from her exercise book and the text book and then to go through and condense these further onto little flash cards. The other likes to make spider diagrams and then to go through these and highlight important points.
If your sons are not sure what they should revise, they should check with their teachers but in our house it's always been all of the the material that they have covered that academic year from the September onwards.
Admittedly though, mine are girls, and all my friends who are Mums of boys say that it's a whole different ball game with boys!