Are you trying to put him down for a nap every hr to hr half?
He could be overtired?
Is he feeding ok?
Have you got a wedge under him so he is on a bit of an angle or tried sleeping him on his side?
I know it is hot but have you tried wrapping him? Maybe just a nappy and a muslin wrap?
Do you have a nice nap routine going on so that he knows it is time for a little sleep?
Just throwing ideas, sorry not much help.
I hope it gets better for you.