If he is too busy to write them, he needs to return the gifts. It is just rude. I had to nag my son to write his, but it doesn’t take long.
Children/young adults need to know how important this is. It is a habit that should be life-long. I have 3 people on my ‘list in my head’ who did not thank me for gifts and they will never get another one.
They were a baby gift, a graduation gift and a wedding gift.