I shall double check with him what he is supposed to be doing as he is not keen on me emailing his teacher!
This raises red flags to me as a teacher- I have had a few emails from parents concerned about lack of work for home learning etc and they have all come from parents who's children aren't completing all the work set.
Are you able to see what work he is being set and check what he has been submitting? If not, I would email the school to start off! We have been told not to chase, as some families are really struggling and we don't want to put extra pressure on them.
It's pretty unusual that the triple vs double decision doesn't seem to have been made by Y10. There's quite a lot of triple only content in the topics I teach in Y10, so for us, the decision is set in stone at this point.
We are teaching new content (although we've changed the order of teaching to avoid practical heavy or too difficult topics) as there's a lot of content to get through in even double science. However, equally, revision of early concepts will be useful- the first topics taught at GCSE generally underpin a lot of the stuff that comes later on, and knowing this really well will help him a lot! It also means that no student will be left behind completely, which is a good thing IMO.
Seneca is great, and there are lots of good videos on Youtube as well. Lots of people like freesciencelessons and primrose kitten, and I think fuse school can also be pretty good. You can also find past paper questions with mark schemes at physicsandmathstutor.com - this covers all three sciences and has past paper questions organised by topic, so he can test himself on what he has learnt.
I'd send an email to his teachers, asking if there is anything extra they recommend he could/should be doing and checking his current work is up to standard.