I liked how you phrased your post in the beginning: "Everything was fine perfectly fine ... he was completly into me"
. But well I am not asked here if I find this egoistical or not, I was asked if ENTPs are prone to emotional lockdown.
No, ENTPs are not prone to stay with one emotion for too long except for sadness. Sadness can be a constant companion if you do it wrong, but being angry with someone or forcibly shutting someone out of ones life, you love thats not possible for too long.
Sounds like he had an idea, maybe its the army thing and he know thinks he needs to cut everyone out of his lifes, cause he could die in the service. Sounds perfectly entp, it has the necessary dramatic touch. What it was, it was definitly a decision of the mind, so a reasonable one, therefore he is able to keep it up.
I would investigate if someone talked bad about you or told the entp to let you go, someone who has some influence on him. This could be a reason aswell.
Other than that, this is probably the biggest downfall of the entp. I have done the shutting out people thing to two woman in my life and I regret it. In both cases I wanted to end the relationship and tho it's never easy to end a relationship, I did it quite abruptly, too abruptly. I back then ended the relationships because I saw no chance for growth. One girl was fundamentally lazy and obviously aiming at getting kids early and becoming a housewife, which is not my idea of a good catch and the other one was to flippant, to socially involved, partly more caring for others than for me; not my idea of a good catch aswell. I stayed with the first for four years and with the second for three years and I hoped they will change, like grow in the relationship but they didnt. And the moment I couldnt stand it, I made the decision to end it. And it is in that regards fundamentally difficult to decide anything. I do not want to hurt people, neveretheless when I do it, I do it bigtime and that cant be the way aswell.
Both girls after me became very slutty and a long time I was guilt-tripping myself because I thought I maybe dmaged them. Nowadays I know that I am not the middle point of the world and that there are other factors influencing the life of others and that it was their decision. I definitly stand guilty of ending the relationships without a warning, but hell I am glad until today I did it. Otherwise I'ld have never met my infp