FGS woman, get a grip.
You know that he reads your whatsapp messages and doesn't reply. Just try to imagine, how that looks at his end. He sees it's a message from you...gives it a cursory glance, does an eye roll, and throws the phone on the bed/desk. Are you okay with that? To be someone's annoying puppy?
He's definitely fucking other women. He lives in London. You are safely stuck in Glasgow, miles and miles away. How very convenient. Absolutely no chance, of you wandering in to a bar he is in, with another woman, is there? Very handy indeed.
Do you think, when he doesn't see you for 3 weeks, that he's going without sex? Paah. As if.
Please, please, stop pandering to this idiot. All of this head space you are giving to him, is time wasted on a non-relationship that will amount to NOTHING. You have horse blinkers on, and they are blocking out any lovely prospective boyfriends that are passing you by.
Go out with friends. Get chatted up. Go on line dating. Sleep with other men.
He's taking the absolute piss.
And regards being quiet at the weekend, but messaging when he's at work....oh dear...this is because he can message at work, away from the prying eyes of whatever woman he is with at the weekend. Wake up!
I'm sorry if I sound harsh, I don't mean to be horrible, but this all makes such a sad read. And I've been there. I now have a DH who adores me, and I wish this for you, and you will only find a great guy if you toss this fish shark back into the sea.
As said above, the next time he reads your message and doesn't reply, try to imagine how that scene actually looks at his end. Then get angry.