N.P.
keep sitting him, frequently, try to set it at routine times - after breakfast, before going outside to play, before lunch and nap, after nap, etc.
Pull ups are a diaper and kids realize that very fast, you can do undies with plastic pants over it and that will work better. Or undies with a diaper over it. I had one daycare boy that worked great commando - no undies but pants - I think cause the undies were tight just like the diapers were but no undies made him realize he didn't have a diaper on so he held it.
It will take 10 months of CONSISTENTLY sitting him before he is trained. At first he will just sit, then eventually he will produce, then he will get it. This is a skill just like using a cup or spoon, you didn't expect it to happen over night but gradually as you introduced the skill.