Hello,my Baby Is 7 Months Old, Since Last 20 Days

hello,my baby is 7 months old, since last 20 days he has a problem of loose motyions at that time he was based on nan pro formula.but after six !months of age my baby has a prob of loose motion .now his docter suggest to change the formula from nanpro to isomil..but i noticed his skin rashes in his diaper part..i think is this allergy due to isomil or otherwise..and loose motion s are continue asbefore