Well if his temps are fine and he has eaten since you brought him home, then my guess would be parasites as those can cause really smelly, runny poop. Like I said if you have a regular vet you use, you can probably stop by and get a fecal container and drop off a fecal to get checked for whatever they charge. Otherwise you will have to take him in for a full checkup which when I had that done for my beardie, ran about $200. You can also wait another month to see if it gets more solid. My red tail had softer than normal and a bit smellier than normal poops when I got her but it cleared up in about a month. I figured it was just new home stress.