Researching is never a bad thing! I'm the same way.
Losi discontinued the Bajas because VW went after Losi for making VW Bug bodies. I'm not sure which Baja you are talking about though because there was the Micro Baja, Mini Baja, and the Mega Monster Baja.
For indoor fun 1/18th or 1/16th scale is the largest you can run and still have fun. And even that is too big for small rooms. At the same time these smaller vehicles are horrible for grass and extremely rough off-road like an unplowed cornfield. Even low 1/10th scale trucks are bad in these conditions.
So first you have to decide what is more important to you - running inside or running outside on grass and rough terrain.
For indoor fun and outdoor asphalt and dirt fun 1/18th is the way to go. Losi's Mini-Ts, Duratrax Vendetta or Vendetta ST, or Team Associated's RC18T or RC18MT are good choices. I have to say I have a Mini-Baja (like a Mini-T, but with a different body and body mounts, faster motor, and better electronics, and an included rechargeable battery) and I love it. I like it even more than my Vendetta ST.
Traxxas 1/16 E-Revo and Slash are also awesome minis, but they're not the cheapest or smallest. And the brushless versions are way too fast for indoors.
And then there is also the Tamiya Tamtech buggy series. They are very fun and awesome looking, but they need a few upgrades (motor and suspension) to make them fun. For your son he wouldn't need a new motor right away. The good thing about these is that you can get them on eBay for about $70 and the only things you need are AA batteries for the trasmitter and a rechargeable battery for the car.
If you want to go for extreme off-road and grass then you will be looking at 1/10th scale. I don't know how much you want to spend, but there aren't as many inexpensive options here. You can look at the Losi High Roller or Traxxas Stampede.
Let us know how much you want to spend and we can help you much better.