Your DS sounds very self-motivated, I wouldn't be put off. No-one is expecting the same grade from a self-taught child as someone who has been spoonfed Latin since they were 9.
Cambridge Book 3, chapter 28 onwards is the approx. equivalent of Latin Prep 3, ISEB level, Unless he has passed here, I would think that the level 2 practice exercise books would be about the right level. It is expensive buying the answer book as well, but its a lot cheaper than Latin lessons. They also have hints (e.g.ablative because of the preposition) so you can feel extremely knowledgeable when you explain why the answer is wrong.
DS has started Prep 3 at school, so has just completed level 2 work, so far he has learnt declensions 1,2,3,and 5 and conjugations 1,2,3,3.5 and 4. Tenses he has learnt are present,perfect,imperfect, future and pluperfect. He is just staring the passive as opposed to the active, but that is definitely level 3 work. Adjectives he has learnt comparisons. I hope this gives you an idea of what level practice book you need, he is on chapter 16 of book 2.
DS is Y8, but at a very pushy prep and he needs to pass a level 3 paper for his senior school, so he will be way ahead of many DCs, particularly those who started Latin in Y7 . Maybe someone can suggest a good website for a school that start Latin at Y7, doesn't password it! and gives good detailed yearly curriculum notes.