I don't think the fact that he's an only makes much difference - you'd have to make sure he got chances to mix with a range of other children even if you had several children yourself, so I don't think it's any more of an issue for an only.
But I think you need to consider why you want to HE him at secondary level - the fact that you and DH didn't enjoy secondary school is not really very relevant - your DS is a separate person and it is a different school, teachers, syllabus, ethos, everything. Don't start assuming he'll not like it before it happens.
Many DC do love doing things at home with their parents, but that doesn't automatically mean they'll enjoy HE, or that they'll continue to want to do as much with you as they get older - believe me your 6yo is not going to feel the same way about everything when he is 10, or 12, or 15...
So, I'd leave the idea on the back-burner for now, and see how things progress. HE is great for children it suits, but school is also great for children that it suits.
I'd also suggest you need to be cautious about "selling" the HE idea to him unless you're in a position to do it in the nearish future - it's not helping him at school to think that it might be "optional", specially if he's already trying to get out of specific lessons. If he has a problem about French, then you need to be either dealing with that with the school, or telling him that school isn't a pick'n'mix and he'll just have to accept that he has to do some things he doesn't enjoy.
The secondary curriculum isn't something to worry about, but his attitude to it might be - as long as he's at school you and he both have to be committed to it, not half-hearted.