I'm so sorry you're having rough patches with your stepson Jewlz
As you know, it's far from easy to be a step-parent sometimes, and I think children at that age can very easily go from one extreme to the other--one moment they love you, and the next they don't want to be anywhere near you. My stepchildren aren't as young as yours, but they have their moments where they want to be here with their dad more than their mom, and vice versa.
And it's especially not easy when you don't have the greatest relationship with their mums--I don't really like my husband's ex-wife at all, and it sounds like the same situation you're facing with your stepson's mum. It sounds like you're doing a good job in 'grinning and bearing it' when it comes to his mum, I know it can be incredibly hard to keep from just telling them off when you see them or have them on the phone (in our case she sees my husband as an easy car ride since she doesn't drive, and a walking ATM--part of that's fair since he does have to pay his child maintenance which he doesn't mind, but she's also remarried so it's not like she's destitute on her own somewhere like she pretends to be).
I think definitely the 'killing with kindness' is the best policy I've found--be so sickeningly nice that if she complains about you, she looks like a PITA
But there's a huge learning curve when it comes to being a step-parent--it's a fine balance between being an actual parent sometimes, and learning to ease up a bit when the biological parents are involved.
Good luck--you and your DH are doing the best you can, and as long as he knows you love him, it'll work out