Bad nappy rash in 1 year old?

[Nightfall1983]

DS is 1 (tomorrow!). He has had persistent diarrhoea for 3 weeks now - the doctor has sent off a sample to be tested. He's never had nappy rash before but developed it at the start of the 3 weeks, it's been up and down, generally under control but the last couple of days it's got really bad, it's bright red and he starts screaming as soon as I start to change him. What we currently do use:
Disposable nappy (usually on cloths but too much poo for that now)
Not using wipes, cotton wool and tepid water only
Always dry bottom and try to give a few mins nappy free when possible
Use Metanium - the yellow one

What else can I do to help him?

I used to bathe DS's bum with camomile and honey tea. We are also cloth users and tried the disposables to prevent nappy rash when it got really bad but never found it made a difference. I found out between kids that some people dust with corn flour to absorb moisture, mentally filed it, then got DD who almost never has nappy rash! A friend used exclusively disposables and her son still had a bum like a baboon, leading me to think that some kids just are prone to it. We had some success with soaking cotton wool in the tea and putting it on the worst patches. Vaseline to prevent irritation?

As I say, some kids just have it. Last ditch solution, could it be fungal? Our HV suggested a spell of using an anti fungal cream. Did nothing for us but could work.

I've had camomile and honey recommended before - it's on the shopping list for tonight.

I don't think he's prone to nappy rash as he's only had it once before (when teething) despite a couple of d&v incidence in the past. If it doesn't clear soon will definitely look into the possibility of a fungal infection.

Might regret this (almost certainly) but he's currently napping with just a muslin loosely tied around him like a loin cloth - he fell asleep during nappy free time and his bottom looked so sore I couldn't bare to put a nappy on him - am sure it'll be carnage when he wakes up but oh well.