NewLevelsOfTiredness, what you say is so so true, and really find myself in your words. As a single mum, who has always worked FT in a very demanding job, I have constantly battled with my conscience to be sure that I made the right decision working FT and then to allow myself to be looked after by my husband, time that is taken away from my time with my kids.
I think the difference is that when you're a single parent, you experience dedicating all your time and attention to your kids and usually seeing the benefits of doing so. A 'normal' family would from the start be used to sharing attention and time. The single parent has to deal with the fact that by being with a new partner, they have to reduce the same time they were spending with their kids, and knowing that bringing someone else in the mix doesn't replace the dynamics of a 'normal' family, where mum and dad both give attention, time and love to the children.
You therefore find yourself trying to do the right thing by everyone, giving your partner the attention they require, especially when you are not yet sharing a household, whilst still giving your children the totality of time and attention they would get in a 'normal' family, constantly feeling that you can't give to either all what you wish you could.
When your children show all the signs that they are happy and well balanced, you think you do a good job of it, but when suddenly they show signs of problems, that could be caused by a lack of attention and time, your instinct is to want to resolve this.
OP, do your SS live with their dad FT? I expect your OH believes that the level of attention he has given you has taken away the attention he thinks he should have given his kids, and that is a contributory factor, if not the reason why the child has attention issues.
If you want your relationship to move forward, I would suggest you show your OH that you understand how he feels and that you are prepared to take a bit of step-back to give him more time with his kids. The question is: could you live with this, that is slowing thing down, maybe not being there as often, accepting that you come second?
Your OH will need to find the way to feel confident that he is a good dad, even when he is thinking about his own happiness with you and that the issues are either not related with the situation at all, and/or that he can still provide his children enough attention to deal with the issue in a positive way.