
id do it gradually,he must sit naturally in the day,so look out for it (carry some titbits) when you spot him sitting say "good sit" & throw him a titbit.
if you think you can go over to him without him moving,go over & say good sit,titbit & gently run your hands down his back saying " good sit "
basically try to get alot of "sit" words in when hes in position.
when youve done this for a while,sit down with a handful of treats,show him them & see what he offers you, you could even try saying "can you do a..........sit?" & see what he does.
id also keep a stand command so he understands that the "thing" hasent totally changed,but theres now two things to do!!!!