Nudge him(stirs but not awake) before the 30min mark to reset his sleep cycle.
We tried lengthening his wake windows at that age, hard when they are getting grumpy but even half an hour of distracting with a toy helped lengthening our LO sleep
Look up Taking Cara Babies. They have all sorts of tips and tricks as well as sleep/feeding schedules. Adjust it on the timing you want to wake up and sleep. Hope you find something on there that works for you guys
I know you said he gets grumpy but see if you can distract him by changing the activity for an extra half an hour. A slightly longer wake window may help to increase sleep pressure leading to longer naps
Best way to increase nap length is contact or motion!