Birty I don't have stepchildren but do have a 7 year old and a 12 year old son, so a similar age gap.
The 12 year old can seem much more loud, rude, cheeky and full on than my younger son - he will backchat more just because of the age he is, the thought processes, the vocabulary etc. And hormones are probably not helping.
I often avoid taking them out as a duo. I have day trips with one or the other only. Together they wind each other up, argue and vie for my attention. The younger one often comes off worse just because of the age gap. When I have them apart, they are so much nicer!
If I was taking them both on holiday, I would plan one where the 12 year old can go off alone to separate activities - he gets quite frustrated having to socialise for long periods with his 7 year old brother and do things on his level. And for your SS it is even more of a issue as you have a 4 year old as well. Or I would plan to send my youngest to a kids club to give my oldest some time alone with us.
So leaving aside your role and responsibilities as a stepmother, I think you need to find a holiday with opportunities for your stepson to do his own thing - teenage clubs, sailing lessons, football lessons etc. This will give you time alone with your younger sons as well.
But the first thing I'd do is sit your ss down with some holiday brochures, just you and him - have some one to one time - perhaps take him out for a pizza or something. Ask him what sort of holiday he would like with you, suggest ones with activites he can do and see what he says. You could include your dh in this as well, but if you do this alone, it might be better as he will feel you are not doing this under pressure from his dad.