Noticed a strange pattern. The son is 2.5 years old, speaks not very w…

Noticed a strange pattern. The son is 2.5 years old, speaks not very well, knows basic words. But lately I've noticed that a couple of months ago he started using new word "slope", he said it clearly, and then stopped, a new word "shoes" appeared, and again he stopped saying it, and now there's "let's go". The words he used before stopped speaking. Recently he started saying "Mom, let's go, give me crane, bye bye". He makes sounds. It scares me that when he has new words, he doesn't say the old ones anymore.