Guys I don't think it's about him not being able to stay on the farm, I think he was literally bait to drag A back to the farm and A probably wants little A gone to protect him from something? This also aligns with my theory that G is trying to "humanize" these hybrids and helped A get away because if the center found out a human turned into a hybrid they would either do more research or start doing experiments on A. I think the letter was bait to drag little A to the farm as a threat to A.