Billy Bob I have a 1900SQ ft two story house, gas heat, gas hot water, 3ton 16SER AC unit, electric range, electric dryer along with all the bells and whistles of a modern home and I have a 100amp service. Been at my current location for over 23years and never had a problem period. Besides everything in my house I have three 20amp 120volt dedicated circuits just for Christmas lights (I tend to get carried away )and just for kicks a few years back during the holidays while the wife was cooking Christmas dinner, the dryer going, most of the lights on, a couple of TV's going and all my Christmas lights on I put an amp meter on my incoming service and I was drawing less than 70amp a leg. So unless the house your son is looking at has electric heat he should be golden.
As you said with minimal info you have this is my best guess.
If Insurance companies wouldn't insure homes with 100amp services most of the homes in my area would not be insured.
RETodd is correct about the two prong plugs, as long as they have not be altered to grounded outlets the home inspector shouldn't say anything about them. Take a look around your house to see exactly how many things are plugged in that have three wire (grounded) cords