You sound like a great parent 
It does seem unusual. I had a resident DSD, and two others that were there every single weekend from ages 9 upwards for 5/6 years.
Their Mum definitely wanted to hang on to being the number one parent, and the 'role' of being Mum. Yet in reality she barely saw my resident DSD for 5 years, at the time when she could have made a real difference to her life. DSD was possibly too loyal, too teenager or whatever but hugely resentful in the end of me so I couldn't plug that gap for her.
I did wonder sometimes if the underlying anger from my DSD was a lot to do with her own feelings about her mother. Everyone around would act as if it was totally natural that DSD hardly saw her mother, who in turn acted as if she really did see her a lot. It was weird, sometimes I felt like the only person who could see how hurtful it must have been to be rejected? It wasn't her decision. DP felt good that his ExW had leaned on him for support, and also couldn't see that perhaps him being out at work very long hours and a step Mum she ignored was possibly not the best parenting set up? It didn't do her any good.
She's living with her Mum now, aged 19, but her Mum would like her back with us, just finds her 'too difficult' I think.
Your step son's Mum never sees him? Now that is hard.You must be having to cope with this big unknown, strange absence. Although part of me thinks, because your step son has accepted you completely as his parent, and there is no twisting or conflicting erratic ins and outs of a Mum that is there sometimes, not there others, then at least this boy has a fairly clear and stable life with you all as a family?