Hiya Ant, congratulations on your new fish
.
Does your mate live near? It could be that the fish is still a bit shocked after all the disturbance it has gone through. Did you put it in fresh water when it got to yours, and if so how did you acclimatise it?
The only other thing I wonder is whether all the water disturbance is a bit too much for it - they dont do well with that I dont think.
Try just rippling the surface of the water, make sure he can get to the surface for air (the top of the tank is not too near the surface of the water), and leave the lights off for today and let him settle.
As for food, I dont think there is any special food for bettas, but I am sure that some of the betta owners will correst me on that.