Please help! This is serious

Ill make this as short as poss but if someone wants more info just ask. I want to leave my gf of 14+ yrs. We havent touched in over a yr. We have an 8yr old son. Whom is not biologically mine u do the math obviously she is a cheater. Bk he is not biologically mine my gf reminds me everytime ive had enough and try to leave that i have NO legal rights to him. Now i kno u will say why didnt i leave when she first cheated (ya i said first i kno of a lot lot of times she has cheated and thats the ones i kno of oh and one of the guys was my best friend). Well all during the pregnancy and the whole first yr of my sons life i was told he was mine, dna prooved difrerently. By that time i not only was in love w this beautiful child but i saw just how selfish and neglectful his mom was tward him and his "real" dad was a bad alcohilic with no job who didnt have time for him. Now multipuly my son's bilogical parents problems and more importantly my love times 7yrs and here we are. i have raised my son basically alone. We are closer than any other son and father i have ever seen and he counts on me only me for everything bk i am the only person in his life who puts him first and truly understands unconditional love. What do i do? Leave and hope what i have taught him will b good enough to keep him safe in life or do i stay and have no life other than him (which i am 100% ok w) but his mom treats me like dirt and uses him aginst me, she still cheats so i havent sleept w her in over two yrs, and obviously i cant have friends around. Im alone and miserable and no matter how much i love my son im afraid i can no longer hide my pain and i kno him seeing me hurt and being talked down to by his mom is going to make him think this is how relationships work and that is not what i want at all. Like i said a lot huh? And thats the tip of the ice burg!!Well what shld i do? Roll the dice and hope ive done a good enough job knowing if i leave she wont ever let me see him again or stay and let this relationship slowly kill me until hes 18?