Hi Helen!
I am a grandma to a high-needs infant. He is 2 months old today. Just like your LO, he's a great baby - smiles, coos, doing his milestones - but very sensitive in nature. - Has a real fussy time at night and when he is ticked, you know it. It breaks our hearts when he cries and cries and there's nothing we can do to help him. Both of my DDs were very low-maintenance so I feel awful that I have no advice for my DD except to just keep on loving him (which she does - patiently and unconditionally).
The other night, my DH and I babysat him for about 4 hours so DD and her DH could get out for awhile. He pretty much cried the entire time - I could entertain him with various things for about 10-minute clips (or feed him, which he does for both hunger and comfort - hates the paci). I finally got him into a light sleep around 11:00 pm (normally goes to bed at 8 till about 11-12) and the second his mother walked in the door, he woke right up, looked at her, smiled and cooed (and immediately wanted her boob...LOL...but could not have it because they had a few cocktails so she had to give him a bottle and that royally ticked him off...but he did settle down for her and once he fell asleep, he laid on her chest and was OUT).
So the next day she sent her DH to take LO to his dad's without DD. That went over like a lead balloon. She could hear the baby screaming when they pulled up in the car and as soon as she took him, SILENCE. Now he does have his fussy time even with her - at about 7pm each night for about 1/2 hour...but he is so attached to his mommy.
I've talked to mommies with older LOs and they see that this behavior improves by about a year old - when they can get into other activities and people. She socializes him constantly with both adults and children so I am hoping that will do the trick.
Hang in there - keep posting :)
Hugs,
Laura