What exactly does he mean by platonic?

He means he wants to have a meaningful relationship without a real physical side. Kissing is indeed physical, but I'll guess he means platonic with regards to "anything major". Be careful with him! The age difference is huge. Should "anything major" happen, he could end up in jail. And, out of respect to yourself, I would avoid letting anything like that happen. The situation sounds iffy. If your parents don't know about him, it would be a VERY good idea to tell them. Put yourself first. Make sure being around a guy interested in girls so much younger is even safe.