I'm sure he will love the 10 gallon. However, in my opinion - they seem to like it better if there are alot of plants to hide in/swim around then if the tank is bare with just one tiny plastic plant. Are any of the plants you bought tall? I find that at night they like to have something near the surface to rest on. If you can do live plants - there are some floating plants that will work. It has been my esperience that will they are OK in smaller tanks, they do alot better in larger ones - more active, more colorful
As for the heater- I have a tetra preset heater and mine can't even heat up a 1 gallon bowl to above 74, so my vote is NAY. I would just keep an eye on it.
If the filter current gets to be too much - most my guys do not like it - you can biy filter foam made for an aquaclear filter and put it where the water flows out of the tank. Actually you can use the foam in place of the filter cartridge that came with it-just stuf it inside the filter- instead of replacing it every month, just swish it off 2 or so times a month in old tank water. it will start to look gross - but thats the way you want it. If you replace the cartridge every month like they claim to do - you are tossing out the bacteria you WANT to keep.
I usually get the one for 50-70 gallon tanks and just cut it, if i need too.
Also do you know about the nitrogen cycle? If not, save yourself the headache and read the stickies about it in the accessories forum. I didn;t know about it with my first betta