Basically me and my best friend both have boyfriends but this guy had been messaging her a while back, slightly inappropriate sort of stuff, and his girlfriend found out... now he's talking to her again and him and his girlfriend are back together.

Now he's messaging me stuff and complimenting us and slapping our butts , etc... is he interested or is he playing us for fools? He's older than us, but only a few moths older than our boyfriends :P

Politely tell him to back off and leave you alone. Set up a boundary with him. He's giving you a bad reputation.

He is being inappropriate and you need to tell him to back off. Allowing him to continue to do this, especially since you have a boyfriend is sending him the wrong message.