The size of tank is perfectly fine so long as you keep up with water changes. MANY forum members have bettas in small tanks and have kept them healthy successfully for years (including myself).
You can always upgrade when you have the time/money/space, but for right now that setup is just fine for him.
Heater-wise you can get 25watt adjustable heaters from most petstores that sell fish. If you can get one, great! The only alternative is the non-adjustable ones that tend to not be so reliable with keeping a constant temperature. So keeping him in a place in your home where temperatures dont fluctuate greatly or is very warm would be good for him.
I use a 10W non-adjustable in my 3 gallon and it works fine to keep the water between 78-80F.
As for the flow, looking at your filter it looks like it's prlly the small whisper filter. In my experience those dont have too strong a flow, and walmart sells aeriation kits (tubing, connectors and air stones) that have a little black connecting piece that also has a dial on it so you can alter how much flow there is through the tubing. Very easy to install, it's like legos but with plastic xD
Its usually the betta's with broader fins that have more trouble with flow because the water pushes them around. But I'd say let your little guy get used to it and see what he does. You may find he uses the flow of water to play in, or to deliver food to him. I have several that dont mind a medium flow filter, and one that even sat with his nose in the outflow like a shower.
But your fish will tell you if something is wrong =)
Grats on the new betta, and welcome to the forum!