Just to confirm what others have said. It is great that Wallington have said your son has a guaranteed place. He obviously passed the test very well. Last year those who scored more than 345 had letters saying they had got a guaranteed place. I think they offered down to about 335 in the end.
You do not need to put Wallington first on your CAF. The first preference system works in Sutton so if Wilsons if your first choice put it first and put Wallington wherever you want it, you will still be offered it if he doesn't qualify for the others. It is very tempting to put Wallington first when you read the letter, but you really don't need to.
We had a friend who put Wallington 3rd. Didn't pass Sutton or Wilsons which were his first 2 choices and was still offered Wallington in the first round of offers.
The school will not know where you have place them. They word the letter in that way deliberately to try and tempt you to put them first.
Congratulations to you and your son.