How old and how anxious?
My son is 6, turns 7 in December. He's autistic and anxiety is a massive thing for him, but he's come on so much the last year or so, it's amazing, although he still has his struggles which we're supporting him with.
I'd say the biggest ones for us are -
A rock solid routine, especially in the morning and at night.
Lots of time doing things he enjoys in a no pressure environment (ie park trips and Lego over groups/clubs)
Books about feelings and us telling him about our days, casually including when we've been worried or frustrated or whatever and what helped us feel better, without it being a big deal
Having "tools" he can use - a pack of calming things (photos, squidgy toys) a comfy space he can go to (his bedroom) breathing exercises/yoga that we practice together.
Just him knowing we're there and we get it. Encouraging him to try things, but not forcing or getting angry if he's finding it tough.