Please Help - Constant Loud Night Meowing!

I have recently adopted a Russian Blue rescue cat, he was left alone for a very long period without food, apart from being very thin, he is a wonderful cat, full of love and fun and I am setting about feeding him up and giving him a lot of fuss!  Only problem I have come across is he doesn't seem to sleep at night, instead he will meow loudly through the night whether I let him in the bedroom or not.  Is there any advice you can offer as to why he is doing this? I appreciate this breed is very intelligent, I have shown him his litter tray and toys and food which he has remembered all and uses frequetly but he doesn't seem to be interested in his bed and I am not getting a wink of sleep.  He clearly wants to be around me a lot and is very affectionate, other than craving attention, any ideas why he meow's so loudly and contstantly at night? Thanks.