>>212423131 (OP)He got cold feet at the very end. It could happen to the best of us. The events afterwards proved his lack of complete faith in Don Vito led to his downfall. This is the situation where 99% is not enough. You're either all-in or not. Tessio's potential doubts were reasonable: after all, everybody gets old and we can slip or misjudge things even if our previous records were nearly flawless. Then he looked around and saw that reality started to crush them with seemingly no way out and so he decided to jump the sinking ship right before it went underwater. He was wrong.
Don Vito asked him and Clemenza when he handed the reins to Micheal and he semi-retired as a Consigliere and treat Micheal's word as if it was his. You can interpret it as a gesture to lease your son credibility and buy him time, or as a sign that the old Don still has a trick up his sleeve. Remember, he and Mike involved nobody precisely because of what eventually happened. There's also a reason why they all thought if there's anybody who'll ever betray them it'll be Clemenza. Tessio ultimately disappointed everybody, but mostly himself by not sticking to the Family until the very end regardless how it ends up.