Potty training?

Anyone started potty training yet? I know it’s a little early as far as the standard 2 year mark most people say. But we got the go ahead from our pediatrician to start a gentle approach. My LO would try to take his diaper off and then sit on his potty chair. We picked out some underwear together and he’s super excited about them and loves wearing them. So here’s where we are and not sure how to get past this -

He’ll go into the bathroom yelling “potty!” and lift up the lid on his potty seat, so I take off his underwear or diaper (whatever he’s in at the time) and he sits. He only sits for a few seconds though and doesn’t go. I try to encourage him to sit back down and go potty and sometimes he will sit, other times he won’t. So we put back on underwear or a diaper and decide to try again in 5 minutes. But he ALWAYS goes pee like 30 seconds after leaving the bathroom. I don’t know how to get him to actually pee while sitting on the potty. He’s done it twice and we heavily praised him and he put a sticker on a potty chart I made, but he hasn’t done it in awhile now so I’m not sure how to encourage him to actually go. Any ideas?