well, i'm not an expert on this but I do enjoy various biking disciplines to be able to take a few steps back to get a better view on your situation. I think if you are really just starting out and especially if you want to spend time with Your son I am thinking these bullets apply:
1. son - child - safety
2. heavy beginner - start with shorter distances
3. sturdy, forgiving bike
4. minding the low budget one will only get one bike that will be fun 12 months out of the year.
Here is my advice: get a used Surly Pugsley Fat Bike, subscribe to some
fat biking resources in your area, maybe download an app like Trailforks, head out to the various single track, snow track, gravel and off road trails around where You live.