Counseling is the very best thing you two could do, if you can get him to go. This is a rough place to be in a marriage, and a lot of divorces happen here.
Many men find themselves vulnerable to the advances of younger women, and that is the most dangerous thing for your marriage. Remember, there are MANY young women who routinely flirt with older men, just to see if they can get him interested, and who would enjoy the financial advantages that an older man can provide.
The fact that you are aware of what is happening is really a plus for you. You can't address a problem that you can't see or acknowledge, so you are probably one step ahead of him. He is most likely very confused about his new feelings of restlessness or unhappiness, and will really need you to help him get through this successfully. I wish you the very best!
I'm no expert but i would suggest talking to him, and/or taking him out somewhere he can get his mind off of things and be happy. Remind him of the person he married in the first place. Treat you guys to something nice. I will pray that everything works out ok for you guys.