@anakskul
Don't forget, even with everyone fighting in the previous timeline, everyone died in the battle and he only barely escaped because Tina sacrificed herself for him. Would they have fared better if he had told them? Probably, but if 20 people is raised to 100 people, it still won't be enough to deal with 2000.
And that is assuming anyone would believe him in the first place. Most likely than not, everyone would make fun of him. Worse if everyone thought he was crazy and pick on him, disturbing his concentration and even sabotaging him, that is assuming they don't leak what he said to outsiders, thus causing the attack to occur much earlier before he can be ready.
If my best friend suddenly call me one day and say, "Hey, I came from the future. Get ready, we're going to get attacked by aliens in 5 years." I would be like, what drugs are you on?
Could he have told his father and Tina? He could, but I can understand why he can't. If he told people and they make fun of him, his geas, his determination would've been set back and his actions would've been hindered repeatedly. Naysayers and teases can hurt more than sticks and stones can. He knows this will happen, and that is all that matters. He prepared everything and lives for this very moment.
Well, there's also the possibility that if he had told Tina, Tina would've wanted to fight with him. And we know how naive and annoying Tina can be. Tina is a glass cannon and he'd have to constantly watch behind him to ensure she doesn't get killed suddenly. Having to fight while having to protect her at the same time would've seriously hampered his effectiveness.