18 month old not sleeping

We’ve had trouble with sleep since forever but my 18 month old Is up every 15 minutes. I can’t cope. He’s up constantly or he’s up for hours at a time. He has one nap a day for about 1.5 hrs ending at 1 ish. Bed at 7 ish. Same routine etc

Some nights he’s up for fun it would seem or tonight he’s up crying. I’ve given him calpol as thought his teeth hurt ( I never usually do that but thought I’d try it) Made no difference.

Last night he was up 1am- 4am! I’m working so it’s even tougher at the moment.

Please anyone give me advice or hope

What am I doing wrong?