After a while things turned nasty and i blocked him, moving on with my life. He got into a relationship but was still trying to contact me and ask to see me.
But you said you blocked him. 
Fast forward literally 5 years later and i keep getting random friend requests from him on everything. I ignored them.
But you said you blocked him? 
You simply cannot keep getting repeat friends requests from the same person when you've blocked them. If it's happening it's because you are allowing it. If someone is getting information about you from your SM accounts it's because you aren't taking steps to prevent them from doing so. Stop being so passive and faux helpless about it.
Now recently I've been getting random private calls. They were at late times( 1am or something) so i knew it wasnt a company or spam. I ignored the calls but suddenly another friend request popped up.
I'm starting to feel a bit like a broken record here.... I thought you blocked him? 
Do you have actual proof that it was him ringing you, just a hunch?
Was it direct to your mobile number or via Whatsapp or a SM account?
This really got my back up as ive made a point to keep my baby of social media so i was shocked when he told me he knew and also felt abit violated.....He stopped contacting me for a month or two but then i was having relationship troubles and posted a sad picture. He instantly tried to call me on private number then messaged me
Right. So you felt VIOLATED (strong word) the last time he messaged you and you responded but you still didn't block him or unfriend him, leaving him free to message you again? Can you take a guess at where you might be going wrong here?
Even if i block him (which ive done for numerous years) he finds a way to contact me or will persist with random private calls.
Apart from the time with your mutual friends phone, are you absolutely sure it's even him? Have you answered the calls?
If you know for a fact it's him because you answered, then just tell him straight to please stop contacting you, he's wasting his time, you aren't interested these days, goodbye. Then stop answering random calls from unknown numbers and block any that arouse suspicion. There's a limit to how many mobile numbers he's going to have access to.
If you have actual evidence that he is using underhand methods to continually pester you by phone or trick you into responding to contact by pretending to be someone else, in spite of being asked to leave you alone, then you can take it to the police. But frankly based on what you've said so far they will laugh at you. It's not their job to be your private security detail because you can't be bothered to tighten up on your SM settings.
All this preferring to keep him in plain site just sounds disengenous to be honest. If you keep him as a friend or a follower on any SM then it's not sending a clear message, is it?
This has been going on for 6 years now. When will this end and for how long will it carry on???
It will probably stop when you say you've blocked him and you have actually blocked him.
I cant talk to my partner about it. I tried and his answer was just to block and move on although i've told my dp he finds other means and methods to get through to me.
Waht do you expect him to say, if you won't block the bloke? Do you want him to challenge him to a duel at dawn, or something?
To pps asking how he saw my post i have not blocked him off one social media. There is no point. He will continue to harass me either way
Well that sounds very defeatist. There are a limited numbers of ways a person can see into your online life or contact you online if you have taken common sense steps to protect your privacy. If you won't do that, then there is zero point in complaining about it, while they aren't breaking any laws.
He is also potentionally keeping tabs on my life somehow and i find that quite stalkerish. Wwyd?
well you keep him on one of your SM accounts so... 
It's not that I don't take stalking seriously. I do. It's not that I don't think he sounds pushy and creepy. I do. It's not that I don't understand it's making you uncomfortable. I do.
What I don't understand is why you seem to have done so little to solve this yourself in the most obvious ways possible, and yet you are wringing your hands about it.