Allowing Jack to get to the point where he has significant mats in his hair can be bad not only for his hair but for his health as well. When the mats become large enough, they can provide a home where bacteria and other forms of dirt and debris can hide. This can lead to sore spots and serious infections as the mats rub against Jack's skin. On top of all this, once the mats become large enough, they can become very uncomfortable when Jack tries to move or lay down. Long-haired dogs tend to be more prone to mats than shorter-haired dogs. The best way to deal with mats is to untangle them as quickly as possible once you notice them.
Unless the mats are making Jack uncomfortable when he tries to lay down or are so bad they pull on his skin when he moves, chances are good he won't even notice them. One thing you can count on though is that when the time comes for you to remove them, he is going to let you know just how much he hates any form of grooming. Keep in mind, you need to take your time and go very slowly so that you don't cause Jack any excessive amounts of pain as you work to remove the mats.
Not only does a dog like Jack tend to not look his best when he's matted, but leaving them to grow in his fur can lead to serious skin health issues. Keep in mind that Jack is not likely to enjoy this process. Be sure to use the right tools and give him a few treats along the way. Regular grooming will help keep the mats from forming in the first place.