I really don't think these outings in the current atmosphere is the way to go, at least not with ds.
Your DS will be much more perceptive of your feelings than what you give him credit for. He will sense that you don't like him (and that before he even moved in), so offering an outing, that ultimately, you are not doing for HIM to please him, but for YOU in that you are seeking harmony in your household and within your relationship. He will know that and if anything, offering to take him could potentially make things worse.
If you want harmony, your first port of call is to try to like him. Focus on the positive, on what he's been through, in trying to see the world through his eyes and show some empathy.
The fact that you seem incapable of understanding why it would have been a good think to thank him, not for going on a family outing (even though you considering this normal doesn't mean that he should), but for the fact that he made an effort, and at 10yo, that's much more impressive than an adult doing so. It really was the chance to show him some appreciation for his own feelings and this totally passed you by.
I do genuinely sympathise as being stuck in a flat too small for everyone, when tension is palpable around, with kids you wish were not there, it must be pushing on nerves and as you say, it is easy to lose sight of what really matters.
It is obvious you want to do things right for everyone, you just don't seem to know how to go about it and make poor decisions as a result. I do think counselling will benefit massively. I wish you good luck and don't despair, it can really get better with everyone considering other's feelings, but the feelings of the kids have to take precedence at this stage.