You could be describing my daughter around same age.
She just finished a little program at her school to empower kids with anxiety. My little girl has separation anxiety at drop off time or before trying something new. We had the stomach issue (in our case it's nausea and she has actually thrown up).
This little program she did was excellent. There was a bunch of kids all with anxiety, nothing serious, but like your son probably - the kind that can be dealt with with tools and techniques kids can use.
So the tools are simple. They covered deep breathing the first session. Kids thought this was great. Called bubble breathing. Like your blowing bubbles into a glass of milk. Deep belly breaths - and it calms the kids.
There's relaxing your body. They did this group activity (then we did as family at home) where you tense up your body, then go limp like a jelly fish. You do this a few times. Kids again loved it.
There's more - for different situations. One activity had them think of their 'safe place' - for my daughter it was home with her family. So she just imagines this while breathing.
Then you come up with a coping strategy.
You list out the fear or worry
You acknowledge it - it's real to kids
You list the worst possible outcomes - for your son it might be he will miss his dad and be sad - feel alone, etc.
You list out things he can do to make him feel better - such as facetiming him, texting, calling, writing him a letter, doing a piece of art to show him later, etc.
Or it could be - I get to spend extra time with mom and she'll give me hugs and I won't feel so alone.
Or even - I can sleep with mom like a sleepover if I get lonely.
It's whatever he needs to feel comforted. And the kids come up with this part.
Often the worst possible outcomes - if you go over them - aren't ever going to happen, and the kids get that. And then they let those go.
Then as they cope - you just remind them that they were able to get through this, hence empowering them.
It's been really helpful for us. They also focus on positive strengths the kids have. So every day they say what the best part of their day was. Things they do well. Things they like about themselves.
The other thing we do with our little one - is she started out with a little pebble that was nice and smooth. We called it our courage stone. I told her it helped me when I was feeling nervous or shy, and I would lend it to her. Eventually she didn't need it any more. I noticed it just stayed in her pocket. But it could be a tiny little toy (even a lego figure). Sometimes a comfort object that they have in their pocket or school bag is all it takes. So maybe dad could give him something like that - so your son could see it during the day and feel reminded of him. Some kids have a little note.
I have found in comforting kids (allowing them to cuddle up some nights) is great. One of my older ones still likes to do that if nervous before a test or if just feeling stressed about stuff. Love is good :)