Diaper rashes can appear fairly rapidly. 30 minutes is a bit fast for severe rashes. Often, the rashes start out mild and not too concerning. Then before you know it they're flared up and severe.
If the rash has small spots at the edges of it, he may have a fungal diaper rash that needs an antifungal cream. If not, it most likely is an irritation diaper rash that are best treated by coating type creams, such as zinc oxide. Keep a thick layer on. Apply with each diaper change. Try to avoid having dirty diapers on for too long. And while the rash is severe you may want to avoid the chemical baby wipes as they can irritate and worsen the rash. Soft cloths work well. Disposable ones without chemical additives can be purchased.
Hope he gets better soon.
Advice and opinions given are based on the information provided and must not be considered an official medical recommendation. The limitations of the internet prevent an appropriate evaluation. Always consult your doctor if there is concern.