Rather than "just having him sit there", take his diaper off and keep a little potty nearby. Once he tries peeing/pooping, put him on it. OR you could set a timer for every 15 minutes until you figure out his cues.
I've not had the health or time to start training my now 18 month old, but I had his brother almost completely trained at this point. He's just now telling us when his diaper is wet/dirty, so I know it's time (combined with the fact he wants to go sit and try after he noticed). I'm hopeful once we start kindergarten in August for my eldest, I can start doing the whole potty shebang with the youngest.
We do stickers, but in the beginning it wasn't much of anything for them, but more for me to know how many times we actually made it to the potty and not lose hope. Now it's a really big accomplishment thing. We have a set of stickers if we are dry at night (larger Disney stickers), a set for #1 (all tiny stickers), and a set for #2 (smiley faces). I try to mark accidents on there so I keep a tally for MYSELF where we stand, and if I am paying enough attention or not.
We can now leave the house without accidents (though there was one this week when my husband and father in law took the kids to the playground), and usually can make it to the potty while out, or when we get home without an incident.
When were were first starting to get #2 in the toilet, DS#1 was afraid of doing it, so we would give an incentive - a treat- if he would go. He knew if he went, he got a big sticker on the calendar, and a snack out of a special box. These are things we don't normally get to eat - smarties, m&ms from the US (ours have carmine so we can't use them), gluten free/kosher marshmallow treats from the US, Pez, small toys or books we pick up here or there. He has a choice as to what he would like to pick out, and it feels like a big accomplishment to him that he gets to choose something himself.
We have had setbacks due to some health related things, and I think if it were not for them, we would have "graduated" last year. Were it not for the surgery we had in August, he would have long been on the potty and I would have been working with the youngest already. Instead, he hurt and didn't feel like trying for 3 months. It was very hard, because we started back at square one, and I had to put him back in undies all day and just hope and pray and keep my eyes and ears peeled. If I wasn't paying close enough attention, we were having lots of accidents. However, I found if I was proactive and timed things just so, we didn't have as many, and he regained lost confidence.
A lot of people say children don't have bladder and sphincter control before age 3, but all babies do, we just train them to not use it, by using diapers for our conveniences. A long time ago (1930s-50s) people had their children diaper free by 9-12 months. It's not impossible, it just takes a lot of dedication on the parts of the parents. It needs both involved, as well as everyone who does any care-giving to be on the same page.
If a parent is not on board, the kids pick up on that and it is difficult. Be prepared to take your son to the potty any time you go, even if he does nothing more than sit and play. It's perfectly fine, as it normalises him going to the potty in the specific room where the potty is, IF he has to go. He sees others doing it, and he will gain confidence he can as well.