Hi,
I've been having the same problem with my 2yr old daughter, except she's getting her diaper ALL THE WAY off (!). For her, we realized it's a development milestone, so in one respect we're proud to see she can figure this out - even under 2 pajamas. OK, so what FINALLY worked for us was to put her diaper on BACKWARD and snug so she couldn't reach back to grab the tabs. At night we also add a onesie, which typically deters her...except this morning she figured out how to unsnap the onesie...
The good news is your son is showing his creativity and development that he's figuring out how to get that diaper off! Try the backward diaper -- also, there are nighttime pull-ups that might work for you, too. Good luck!