Do you try and give him nappy free time? Sometimes when changing my Lgs nappy I just leave her nappy off for a bit and make a fuss of her. We play peek a boo or sing whilst she's just lying there. Also we lay her on a towel and after wiping always make sure she's dry by patting her bum with a terry towel
Hi! My LO had the same thing and my doc gave some Zinc supplements and we applied a lot of Sudocrem.
I find naked time really helps as the air helps. We bought some incontinence pads on Amazon and pop LO on them on the floor or on the bed
When my little one had sore bum due to so many poos with an illness Dr advised to use thick layer of sudo cream. I also used metanium which is a treatment cream. And lots of free nappy time defo helps! It’s risky business but always put him on a changing Matt and large towel around him! Usually best straight after a poo ! Hope gets better soon ! X