If your son is a keen reader, his spelling will gradually improve, especially if u can encourage him not let worries about spelling prevent him from writing what he wants to say.
Many schools (very sensibly) don't have spelling tests any more because they tend to be of little help in improving the spelling abilities of poor spellers. Doing as much real writing as possible is much more useful for that.
Spelling tests give naturally good spellers opportunities to shine but are a nightmare for poor spellers, without helping them to improve. The latter tend improve most by learning from their own mistakes, by the old LOOK - SAY - COVER - WRITE - CHECK method, picking out just a few words at a time, again, and again, and again.
Because so many English words contain one or more illogical letters (e.g. mANy, English, wOrds, OnE, morE ) they give some children a very hard time. Good spellers are generally simply lucky to be born with a better visual memory and are able to imprint the right look of words on their minds more easily, without having to work at it. So they love spelling tests, but most children they are largely a waste of time.