it could mean a myriad of things. i wouldn't worry so much about why he wants to slow things down; because that'll come out later. you have to trust that.
what i would recommend you do, which is probably the hardest, is to give him time and space. that part sucks, but if you keep contacting him and pushing him for reasons, etc.; then he will probably bolt.
most men (myself included) internalize problems we have. we hold them in and work on them internally until they're solved. once they're solved we can talk about them.
i would recommend you have a talk with him. let him know you don't fully understand why he wants to slow things down as you felt things were progressively going so well. i would also recommend that he initiate the majority of the contact (phone calls, texting, etc); but let him know that's the intent.
let him know you want things to progress and work out, and if he needs to slow things down to figure that out, then support him in that.
it's definitely harder for you to do that; but picture a time where you'd want to figure something out, and would want him to give you some space...
- Response by boggs77
, A Guy Critical, Male, 36-45, Houston, Technical