Are you sure he's not teething? DD usually has acidic poos when she's teething which gives her nappy rash. She's only had non-teething related nappy rash once. We use Sudocream which works a treat. I actually put a small bit on her with each nappy change which may explain why she hasn't really had much nappy rash (besides when teething).

It could be the cows milk so I guess returning to just formula might answer that question. It sounds like a good idea to wait until he's 12 months before trying cows milk again though. DD had cows milk (a full bottle, not mixed with formula) for the first time at 12.5 months and had no problems at all.