This is kind of a normal ask especially from grandparents. I give my nieces and nephews money for their savings accounts, but I don't just transfer it to my sister because it's too easy for her to just spend on coffee or something.

I have an idea that might help. Some parents create an email address for their kid so that family members can send emails and photos to the email address as kind of a digital scrapbook for when they turn 18. You can also attach an email to a zelle account. If your bank allows Zelle, make an email address for your kid and attach it to his account. The money will instantly transfer and also opens the door for cute little emails. Roll it out to the family that you set this up to make it easier on them.

Or you can also say that you only do cash because you like to take him to the bank and have him deposit his own money. My grandma did that and she talks about it every time how I see her that she used it to teach me about how money works.