It could be the heat aswell! Or like you said a growth spurt. I'd just keep an eye on him and make his feeding doesn't decrease. Has he had any recent jabs or anything?
Mine would have days like this too. If he’s having plenty of wet nappies and feeding well, just let him sleep. He’s probably doing a lot of growing and just needs his rest! They go through so many growth spurts…take the sleepy cuddles :)
I would say normal - my LG had days like this when she was small where she'd legit just seem to sleep and nap all day - I assumed she needed so let her x