Oh dear....i feel for you. My DD was a shocker when it came to day sleeps (she'd sleep well if we were holding her but not if we put her down). Trying to think what we did....

We used to wrap DD at night but she didn't like it in the day so we put her into a sleeping bag in the day and that helped.

When she woke after 30 mins, I used to go to her, turn her onto her side and tap her to sleep..sometimes it took a long time, but once I got her back to sleep she would sleep for ages after this.

My DD hated sleeping in her room during the day (no problem at night but not in the day), so we used to let her sleep in her rocker in the loungeroom (until she started to roll), as long as she slept i didn't care where! Although I realise that this could be difficult with a 19month old.

Good luck, hopefully he learns to sleep in the day, for him and for you!