My son is 2 & he has had a rash in the crease of his thighs on either side of his testicles for the last few days. I tried diaper rash crm, not workin?

Use other. Sometimes other creams work better. If still not improving may need prescription, to be seen by doctor.
Likely fungal. Try antifungals, may be yeast or tinea cruris. Lotrimin or Lamisil (terbinafine) topical every dayor twice a day. Keep area clean and dry. Tinea may take several weeks to clear.