Where Did He Learn That?!

A few nights ago during this 90+ heat wave, Russ and I took the kids down to the beach after dinner. Aiden was hesitant to go even half way down towards the water, and I saw some small kids playing. So I asked him if he wanted to see the little boy. We were able to get him to at least get towards the somewhat solid sand, so that we weren't stepping six inches down. We were playing with bubbles and sitting on the sand, when a little boy (turns out he was 9) started to say hi and just kept looking at us. He wasn't sure how to engage. So I asked Aiden to say hi back, and I explained that the boy wanted to meet him. With that, he stepped forward holding out his hand! He wouldn't say hi, but he wanted to shake his hand! The little girl that was with this boy came over and said hi as well; Aiden proceeded to also shake her hand! It was so sweet, and Russ and I sat bewildered wondering where he could have learned that. We have two theories- Russ says he shakes his hand as part of their hand shake (I have never seen this handshake). My theory is this book: