The 1st rash you described is very typical of urticaria and you were prescribed the right med.but what you are describing now is a different kind of rash.
Next Steps
consult a dermatologist till then take an antiallergic and apply atogla lotion to soothen itching
Health Tips
make your child wear loose cotton clothes and find the reason for his frequent skin reactions.mostly there is an offending factor (eg.clothes/towel/sea food / preservative/pool water etc.)