Have experienced both in my family and all I can tell you for sure is that convincing someone to get a diagnosis is only a tiny bit of the battle.
There is no guarantee they will get help, and the disorders themselves, mean your unlikely to ever know anything for sure.
About anything.

My brother s like this, I watch him wrap his girlfriend's round his little finger, with all kinds of stories.
First thing he says is
"Dont believe my family/friends if they say bad stuff about me".....(followed by a sob story)

It's ALWAYS the other guys fault, ALWAYS.
'His ex's are all crazy, so don't listen to them.'
(No,Most are just mad cos he ripped them off, or slept with their sisters)

He does have wonderful traits. He is amazing with his nieces and nephews. Generous to a fault, usually with someone else's money.)

He isn't a bad guy, just a God awful boyfriend.

I wish you all the luck in the world, remember to always address your own stability and mental health first.
That's my biggest piece of advice. Then think about if you can love him as he is. Imagine nothing changes, imagine it stays the same. If you still love him and accept him then it will be down to you to make things work, by not getting drawn into and hurt if he lies to people or makes up stories, etc.
All the best.