Son is strange with other adults

M💙

How can I teach my 8 month old son that it’s ok to be with another adult? If he doesn’t see his grandparents in 1-2 weeks he cries and reaches out for me as though he doesn’t recognise them and they are total strangers.

We took him to a swimming lesson (more safe swimming for babies rather than full on swimming up and down the pool) and when the teacher took him he started crying and wanting to go back to me. Anything I can do to help this?