Hi sausj and welcome to the forum.
Is this the first time your betta has ever done this and is this the usual routine you go through when you change his water?
It sounds like a sudden development of Swim Bladder Disorder for some bizarre reason. Normally this is caused by poor water quality but that's obviously not possible seeing as you had just changed the tank. It's also caused by injury to the swimbladder. Was your betta in the tank while you changed the water? I have noticed with my own guy that sometimes when I am pouring the water back in, it creates a strong enough current that he is tossed around out of control for a moment. During this time, if you have any decorations, your betta could be tossed into them.
Secondly, what temperature do you normally keep the tank at? Do you use a heater? And what temp was the water you put in? Was it very different in temp from the water that he had been in prior to the change? To minimize shock, you'll want to replace tank water with water of the same temperature, give or take a few degrees. The ideal temperature for a betta is around 78-80 degrees farenheit. For most people, they can only maintain this temp with a heater (unless they live in very tropical countries).
For the moment, I'd suggest putting your betta into a smaller container for treatment. The cup he came in from the pet store (if you bought him from one) is ideal because it can be floated in his original tank and he can stay warm. Get a 1 g container and mix up dechlorinated water with 1 tsp of epsom salts, the same kind of salts we humans use to soak our sore muscles in. Pour some of the mixture into his hospital cup and then put your betta in there. Keep him there for at least 3 days with the salt and change 100% of the water every day. If he shows no sign of improvement, continue for another 7 days with the salt treatment. Also, watch the bottom of the cup for any waste because another common cause of Swim Bladder Disorder is constipation.
Fast him for the first day or so, then offer him several very small meals, like one pellet each time.
I hope this helps.