Nappy rash & possible diarrhoea

Hi ladies, Any advice welcome please? My son turned 1 on the 18th and we started the transition onto cows milk from formula. He drinks it really well so we upped his bottles to half cow, half formula. He usually poos 1-2 a day but since the increase in cows milk, he’s going 4-6 times a day. First thing when he wakes up is ‘normal’ and then the others are basically yellow water and now starting to get mucusy. We put him back onto just formula as we guessed it was too much dairy. It’s been 4-5 days of just formula and he’s still pooing so often the yellow water and his bum is so so sore. He’s never really had nappy rash before and it’s now to the point where the skin has peeled a little. He’s so red and blotchy. We’ve been using metanium cream, very gently wiping him (we use Huggies water wipes), we use pampers nappies at the moment (used to only use these at night for about 11 months now) and I’ve also now been using sudocrem nappy rash treatment cream (instead of the barrier cream) over the last couple of days. Can anyone think of what else could be causing the diarrhoea type poo & nappy rash? Or advice on how else to speed up the healing? Feeling so bad for our boy as he’s wincing during a nappy change when we have to apply new cream. I do also plan to call the GP tomorrow.