Will children lose the ability if we don't follow it on a regular basis?
I don't think so.
I think, sometimes it is necessary to review words especially if you retire those words more than a month or two. Just show and read it to him as fast as you can or you can put these words in a sentence for a different approach.
Sometimes kids are not just in the mood to show us that they already knew the word.
Kids can't focus sometimes. Sometimes they are just preoccupied of the things that they wanted to do, sometimes they are just hungry or not comfortable.
Try to review those words to him in a day or two, once or twice a day. Make sure he is comfortable, in the mood, not hungry and happy. You will see that you don't need to start all over again.