Overheating after replacing thermostat

Sounds like you have an air pocket. I would recommend starting with getting a funnel shown to ensure you are pulling coolant in when the air is working itself out. You can get these funnels at most parts stores.

Here is a video that shows how to use it:

https://youtu.be/7n5zE4ObQeE

The other thing I would try is while you are doing this, jack up the corner of the vehicle closest to the radiator cap. This will cause any air that is trapped to work towards the highest point.

Replacing the thermostat was definitely the correct repair direction for your initial slow to heat up condition. However, based on the fact that the engine never overheated until the new thermostat was installed, I'm lead to believe that it may be defective. I would suggest having the same shop who installed it, remove it verify that it's opening properly. That's really the only thing that makes sense at this point. I've attached the thermostat replacement procedure just in case you need it.

Let us know how you make out!

I would follow KaseKenny's instructions regarding air pockets. That certainly is a possibility. Have your son take a look at the instructions I attached to verify that the thermostat was installed properly too.