One thing I have noticed is that people tend to make quick diagnoses with few facts. The next step is the new snake owner who is already worried and trying to do everything right goes into a panic. “OMG what should I do”
Three months and 7 grams is “small normal”. It is not to be worried about. Make sure he is being fed on a regular basis and has all the other requirements of life and he will be fine.
At that size he should be eating a small pinkie, about 1.5 grams, every 5 days or so. Try to be consistent with your feeding and I think you will see quick results. As MerlinsPop said, he may have been small to start with. It may be the previous owner didn’t feed it regularly or often.
Corn snakes are tough. If you have done your reading and are trying your best, your critter is going to do great.