feel like im losing my son

[dispondantdad]

first post here, so sorry for it being "unpositive and long"

my son is 12.
Me and his mum split up when he was a baby, what followed was traumatic and almost broke me. Constant visits to court for access, turning up to get him on my visiting days and nobody being there, etc etc.
I never gave up, I just fought on and eventually a routine formed. I actually ended up having him nearly as much as his mum, mainly because it suited her life style at the time. I had him most week ends, every bank holiday sunday, every new years eve. He was with me full time for a while because he disliked her boyfriend.
i had a great bond with him, and even though he had his moments, and was a handful for his mum, i was able to get him back in line as soon as he was with me.
hes never really known me have a proper relationship, i had girlfriends but chose to keep them separate because i knew there was no future in it. However, 18 months ago I started a relationship and for once was happy and decided to pursue it (we're still together). Theres history between my sons mum and my girlfriend, long story, but my girlfriends ex husband and my ex had a brief relationship about 7 years ago (yes ,complicated). My girlfriend was never nasty about it even though she had every right to be, but my sons mum has always been horrible about her (its how she is).
Ive tried to include my son in the relationship, but in the back ground is his mum, putting things in his head, e.g, "your dad doesnt bother with you as much now hes seeing her, your dad thinks and does more for her kids than you" etc etc.
He was with me the other morning and asked for his dinner money, he wanted £7.50 to cover the 3 days he was going to be with me, I only had £3.80, but said id give him the other £3.70 the next day. Apparently this wasnt acceptable to him, and unbeknown to be, he text his mum, next thing shes ringing my house (ive blocked her from calling or texting my mobile), I wasnt happy, took the phone and hung up. That evening, he started going on about dinner money again, saying I should give him £10, then he disappeared upstairs, then reappeared saying he was leaving because his nan was coming to pick him up (her mum).
I let him go but rang her and asked why she'd agreed to pick him up when hes supposed to be with me, she said because she was concerned he had no dinner money, I explained he did have dinner money and that she had no right to undermine my authority. Shes always spoilt him and he knows he can play me off against her and his mum.

So, im upset, that hes purposely caused problems where there wasnt one, that hes developing a generally bad attitude, that I can no longer reprimand him because he just goes running and gets back up, from the other family if i do. Ive lost the bond and respect he had and I fought for. Part of me thinks hes just unfortunately inherited his mums personality and theres nothing I can do about it. I know part of him is angry because he was so used to there just being me and him and now he has to share me, but I have a right to be happy and with a person i love.
I dont know whether to just step back for a bit and let him decide. Im exhausted by it all and I dont even enjoy my time with him anymore because of his moods.

Advice appreciated.

[evenafterall]

You need to stop thinking its a two way thing. He's a child. You are his father. His relationship with you will shape him for life. It's your duty to look after him and give give give and expect nothing in return. That's your role. Everything else is beside the point. Kids get moody. His situation probably looks impossible to him. Establish reasonable boundaries re behaviour but lower your expectations. Relationships grow and change but I really think the onus is on you to nurture him.