Thank you for your reply, I will check the water.
I have lowered the water level, it seemed to be a lot of effort for him to swim to the top. Was that the right thing to do?
I wouldn't say it was necessarily wrong, because he may appreciate having an easier time getting to the surface. However, if there is a problem with the water quality, lowering the water level will cause toxins to build up faster because there's less volume of water available to dilute them, so I would keep on top of those tests to be sure the water quality is what it should be. Another drawback is that more frequent water changes are required when you have a lower water volume, which can equal more stress for your fish.
That said, if he's having trouble getting to the surface, there are a few things you can do to help him out in addition to or instead of a lower water level. I'm not sure what's available in your area or what types of plants you have, but in my experience, bettas who are having trouble swimming really appreciate broad-leafed plants (real or fabric aquarium decorations) as places where they can rest near the surface.
There are also two products from ZooMed called the Betta Log and the Betta Bed Leaf Hammock that might be helpful. The advantage of the leaf hammock is that it adheres to the side of your aquarium by means of a suction cup, so you can adjust its placement to meet your fish's particular needs or preferences. The log is great because it floats at the surface of your tank and has a hole at the top, so your betta ran rest in it and need to go up only about an inch or two to breathe or to eat. I have seen bettas who are weak due to old age or illness make good use of these logs. I think they may also feel a little safer because they're sheltered on both sides and at the bottom.
One complaint I have seen about the leaf hammock is that some of them are poorly manufactured and start to rust because the wire that supports the leaf isn't properly covered, but I have never had this problem with the ones I've purchased. The only problem I've had with the log is that the paint tends to peel off under a heavy current or if there's significant algal growth on the log.
Sometimes, I'll use a net breeder to help bring a weak fish up closer to the surface, but I don't like to do this because some fish get stressed out by being confined in one. If I do have to do this, I'll try to add small plants (real or fabric) in with the fish so they have some cover to help them feel safer.