My ds has just learnt how to pull himself to standing. Now everytime I put him down for his nap, even though he's tired, he just stands up and plays with his cot. I have to go back in repeatedly and lie him back down. Sometimes he finally goes off, sometimes I give up if we have to go out. He doesn't do it at night, only for daytime naps.
My boy did this a few weeks ago at a similar age. I did the same as you but then when he looked really tired (rubbing eyes etc.) I would sit by the cot with my hand on his waistband so that everytime he tried to roll over, he couldn't. He got a bit angry at first but would usually be fast asleep in 5-10 mins (this was during heatwave so couldn't go out!)
Th good news is he stopped doing it after a few days. I mean he still does it occasionally but one or two lying him back downs now does the trick!
My little girl has recently started doing this too.... i have found (so far) that if she is tired i can leave the room and she will lie down after a while and go to sleep. If i stay in the room she tends to stay standing and want to play. So i guess it depends on how well he self settles and whether youa re comfortable just leaving him for a while.
Thank your lucky stars he doesn't do it at night. When mine learned to pull up (at 5.5 mth FGS) he woke up every 30 minutes and stood up and yelled. And I was working full-time. It's a phase, it passes.