My 7 month old has has diahrrea x 3 days, no fever, slightly more lathargic than usual, eats ok now, what should i do?

See a doctor: Although the patient is reported to be eating well and is apparently not vomiting, the increased lethargy warrants evaluation by the child's Pediatrician today

Continue formula: As long as the baby is taking in enough fluids that the diaper gets wet every few hours, you can handle most diarrhea with patience. If he refuses to drink, develops fever, vomits frequently, or keeps crying, you need to take him to see the doctor the same day. A baby under a year old should still be having formula or breast milk , so don't replace those exclusively with pedialyte, or similar.