A healthy betta fish should be able to easily escape the pull of the filter. Sometimes he may swim too close and his fins may get sucked in, but he should be able to get away with no problem. In most cases, it freaks them out a little and they learn to stay away.
So, if your betta keeps getting grabbed by the filter, I would first be concerned about his health. Look for signs of fin rot or other indications that the water quality might be poor. Check for signs of disease. Test the water and see where the parameters stand.
Getting caught by the filter once or twice may have caused injuries to his fins, which would only make things worse.
The other possibility is that your filter is just too strong, and he's getting worn out trying to keep away from it. See if you can take some steps to reduce the flow, or get a low-flow filter. He should also have a little hiding spot where he can get away and escape the current when he needs to.