18 month old boy and potty training?

When my first was 15 months old I started potty training him. I just took him to the bathroom every 30 minutes.

When he does a number. 1 or 2 in the pot.

He's asking to sit... let him sit. If he wants to sit with out a diaper, great! If he wants down, let him down! As long as there's no pressure, there's certainly no harm. We followed our girls' cues like this.. Once he's comfortable (and if he is now, start now), then start putting him on there at certain times of the day... maybe start with first thing in the morning (if that's okay with him) and first thing after nap, about 15 minutes after meals... Most likely, if he likes sitting there (and a minute or two is fine) you will start getting some "hits" without even trying. My daughter started asking to sit at 15 months; at 16 months we made it a regular thing. By 16.5 months she was only pooping in the potty! This is all still in diapers. Soon she started being dry after some (long) naps, so we just switched to undies at 17.5 months. We cleaned up some accidents for awhile, but by 20.5 months she was doing it all on her own.

Also, reinforce his successes with praise, applause, hugs... that's really all they need to feel that what they did was okay... and if you need to offer stickers or small treats later, like when you're trying to get him to tell you when he needs to go, you can... but often that part isn't even necessary, especially for those ones who catch on before two, an age where they get such a thrill from making you happy!

I let him set whenever he wants, I just get confused as to whether I am supposed to take his diaper off every time or just let him sit for as long as he wants. He likes flushing afterwards too lol.

I would pull his diaper down when he wants to sit as long as he likes sitting that way... And that's great if he likes the flush... once he starts peeing and pooping in there, you can make that his reward! If he's a long sitter, then definitely try the post-meal thing... and bring a few books if you want... I bet you will get a b.m. quickly, we sure did.