Tabby is afraid of becoming domesticated. He believes that will destroy their relationship, their love. He believes he has to go If I go... I stay me. I come back, and I'm fresh. I'm new. Like a tuber lying dormant underground in winter that sprouts anew in the warmth of spring.
Alvin is afraid that Tabby won't return, so wants Tabby to carry part of him on the journey, so Tabby will remember him. Alvin is only now glimpsing Tabby's idea of renewal.
Neither will forget the other!
Of course next year Alvin will have satisfied the requirement in Granny's will to stay for a year in the house, so they could go South together. Granny's initial note mentioned cash that could go to the kitty shelter, and we heard she went to Florida for winter. She may have been richer than we realise. I can see a future with two young guys wintering in Florida.