I think for some babies it is just 'them' kwim? I remember when I was 8, my baby cousin was always a very jumpy baby and quite easy to upset, so natually my brothers and I took great delight in making him scared/upset and even making him cry - and all we had to do was shout "BOO" at him and the bottom lip would start quivering and that would be the end of him. This started when he was only a few months old and lasted right through until late toddlerhood. But it got the the point where he would totally over react in a situation too and his mother (my Aunt - who was also a naturally nervy personality) would mollycoddle him to the point where all loud noises, strange voices/people would upset him as he became so sheltered from them in his day to day life. he was also her first child.

I am not saying that by comforting your child you are going to 'ruin' him as what I have told you is based on my own personal experience of it and was merely to provide an insight, but in my experience with my own children I have learnt that sometimes yes, they need to be comforted when upset, but they also need to learn that whatever it was that upset them wont hurt them. My eldest was deathly frightened of loud noises, which was a catch 22 for a long time as he loved watching tractor pulls, car races etc but we would have to go armed with ear muffs to dull the noise. All three of my children have at some stage been terrified of their uncles (my brothers) as they are quite big sollid men with loud booming voices - but now they are fine with them.

Unless your Dp is doing something to deliberately upset/frighten your little boy, then it could be a case of him just needing to learn that this is how Daddy is - sometimes loud and sometimes scary kwim? (but loud and scary in a good way) And in no way do I mean for you to expose him to a situation that is obviously terrifying him, but he wont ever learn to be comfortable around his father if his every moment with him is guarded.