If you read the Op with a bit of empathy, Oblomov, it's blatantly obvious that the OP has some significant mental health issues (PND, anxiety, depression - doesn't matter what "brand" it is.)
OP you've acknowledged that you're BU. In order to help your anxiety (but not feed your irrational fears) can you have a chat with DH, be really honest about how you're struggling with this - which is not his fault, but the fault of your current MH issues. And agree some actions that will help you feel reassured throughout the trip. For example, a video call every evening at Xpm so you can say goodnight to DS. (Which is not unreasonable at all - my son is 23 and I still call him every other day if I'm away from home!)
However, you also need to be accepting of any requests he has - such as not bombarding him with messages every hour asking how DS is, how did he sleep, what has he eaten, etc., then sending increasingly panicky follow ups if he doesn't reply immediately. That kind of activity will feed your anxiety goblins and put them further in control.
You said you were going out the first night but then I assume you'd be alone for the 2nd? Get some stuff planned so that you're not sitting around feeling worried and lonely. Invite a friend round. Plan a long, luxurious soak in the bath whilst drinking wine and playing Netflix on a laptop/ipad/phone. Cook yourself a meal that you love but DH hates. Go to the cinema (alone or with a friend.) Schedule a long catch up call with a friend or family member you haven't spoken to in ages.
Write down a list of reasons that you don't need to be anxious.
"DS is with DH and he would never endanger our son."
"I know where DH and DS are. They are fine. I will speak to them at 7pm."
"I do not need to be anxious or seek reassurance about DS's safety. DH and DS are safe with [friend]."
"If anything bad were to happen, [friend] would call me immediately."
one thing you must impress on DH is that if something DOES happen (e.g. DS falls off swing and grazes his knee) then you need to know about it that day. A) so you can reassure DS and say "Oh dear, did you get a poorly knee? It will soon feel better and I bet you had fun before that, didn't you?" and B) it will stop your anxiety goblins from screaming "He's come back injured!! You're a terrible mother for letting him out of your sight!! This must never happen again!!"
Hope you find some of this helpful OP. Anxiety is a horrible bitch. 