I have a similar issue. My knees get very hot after lots of biking/snowboarding/running etc. On the bike after a while hill climbs hurt my knees and they feel a bit like they are grinding. Walking up stairs is also slightly painful.
I went to see a specialist and had an MRI scan for a suspected cartilage tear (my knee also clicks). Turns out the inside of my knee cap is rough instead of smooth and this causes the friction. I was told it is common in teenage girls (I’m 28 year old male)! The doc told me just to live with it and take painkillers when required.
I now take ibuprofen at the start of every ride which helps and spin lower gears on the hill.
I would suggest he goes to see a specialist or physio.