Sounds like it me. From your post it seems in his first 5 months, he has never had you be gone from his side, so you have always been there.
I would start giving him time alone, in a location he feels safe. Maybe the crate, and you just go out of sight for a spell. Wait for him to settle down then come back to let him start to know that if you go away, you really will come back. It might be a long pathetic sounding issue too, so be prepared. My current little monster, Sunshine, would sound like she was being torn apart and tortured when I went through this!!! I put her in her enclosure, told her I had to go and would be back later, not making a HUGE fuss about leaving, just telling her. I would go outside and she would SCREAM and CRY for over an hour!!! The sounds will tear your heart apart and you will feel like the lowest form of life on earth putting such a little puppy through this!!! After she would tire herself out and settle down, I'd wait a few minutes then come back in. Again do not make a HUGE fuss about being back!!! Just a greeting and then outside for bathroom and immediately distraction with a game or something fun. I did this with Sunshine long before she was even close to ready for sit/stay.
If your boy already has a good 'stay' you can build up to this by having him sit/stay and go out of sight for a few seconds. Come back in Good Boy! and repeat regularly. Slowly build up to where you can be gone for a few minutes and he will still wait.
Will this cure it for life? Maybe not. At 2 1/2 now I can leave Sunshine in the car, with windows 100% down and have 100% confidence she will not leave the vehicle, 100% confidence. I have had reports from folks that she sat in the drivers seat and whined the whole time I was gone. I have other reports that she sat in her seat (passenger) and just stared at the door I went in. Same with at home. I have left for work and she will still cry for 30 minutes after I close the door. Other times she will hop right up on the bed and go to sleep.
Best of luck, just keep at it and he will eventually catch on!!!