I have suspected that DS (who will turn 8 in a few weeks) has some anxiety for awhile. He is very sensitive and we don't watch the news or the weather channel because he can fixate on something and starts to worry/repeatedly ask questions/etc.

Over the past month or so, he has gotten frequent stomache aches--usually at bedtime, first thing in the morning and sometimes in the car. He's had lots of issues over the past few months--horrible stomache virus in November and an appendectomy in early December. He has checked out fine physically. Yesterday, his teacher called and said that he was clearly in pain. I picked him up and took him to his doctor who says she suspects anxiety. Last night, he told me he got sick in class because his classmate explained "Five Night's at Freddy's" to him. (I had to google this). He had trouble sleeping last night and looked exhausted this morning.

Any advice? Book or article recommendations? Thank you.

Anxiety is a tough one - but there are some tools and techniques you can teach your DS that might help.

Anxiety BC is a good place to start - they have a good explanation of what it is, as well as tips, techniques, and so on.

https://www.anxietybc.com/parenting/parent-child

There are books for kids about anxiety, but these didn't work for my DD (she felt they talked down to her).

For sleeping, we've used various things - lavender sachets/spray, a relaxing recorded 'sleep story', red colored night light.

(Sleep meditation we've used is called Bedtime Meditations for Kids - it's an Android app - but there are others out there too).

We also ended up seeing a psychologist (DD refused to go, but us parents went to get advice on how to work with DD and talk to her about her anxiety)

My son has anxieties - every since he was born ... imagine the worst separation anxiety you can imagine ... it was worse ;-) - now he is 9 and still worries - about many many things ...
He also suffers from stomach aches which are part of his Migraines. When he was little he only had the stomach aches - inexplainable horrible pains ... around age 8-9 he had the first headaches.
I have migraines my dad had migraines - all my siblings get them so I am not surprised but a lot of people have never heard about " intestinal migraines" so it is sometimes a bit hard to explain to a school ...
There are some awesome resources regarding anxieties online but I would strongly recommend getting a referral from your pediatrician to a good child psychologist. Anxieties are very treatable - and chances are that they will not go away on their own.
I think we are making some progress with our son - he can now recognize anxiety and he is starting to learn coping mechanisms. We have described Anxiety to him as his " worry glasses" that he needs to take them off to see a situation with his " real eyes" - we always talk about situations and assign percentages " how likely is it that your principal will expel you because you lost your paper" or " how often do you think tornadoes hit in this area"? Is this a realistic fear or do you have your worry glasses on ... it helps ..
According to our TAG teacher many high functioning kids suffer from anxiety ... you are not alone !