Not all chat rooms are ‘sexual’ I used to go on one from a local forum and it was, literally chat but it seems you have other suspicions and have had them most of your married life
Your question ‘should I be concerned’ is really impossible to answer as you seem to have no idea what kind of chat rooms he’s on, if they are sex chat rooms or chat rooms looking for a partner etc you really do need to worry If they are chat rooms belonging to a hobby or interest may be not
But I think there is a lot more to this and chat rooms are just the catalyst
Do you love him? Is the rest of your partnership good do you do things together, talk, have shared hobbies, take walks etc in other words have a good life together
Lots of questions to ask yourself