Hey Mak
You need to figure out if this is just an itch. If it isn't, then you need to figure out if you have some type of future with him or not. Long relationships can fall into complacency, but I am sure that your partner has many things that are comfortable and comforting to you, first flush excitement can make you forget all that.
List the things you would have to do to be with him. Properties, cars, investments, pets, do you share any of those things with your partner?
If you do, are you prepared to lose half of those things?
Are you prepared for the fall out from both your family and his?
Can you bring yourself to tell your partner?
If you answer those questions, and the answer is still yes, then be responsible. Don't sneak around, its a really bad way to start any new relationship. If you really want to be with this person, you will probably need to live alone and get to know him properly, minus the glamour of forbidden fruit.
Hope that makes sense, I have a hangover.