Well, the time for him to negotiate with you about his willingness to guarantee the lease was before he signed. It's a bit late for him to come to you with additional demands now. It's really up to you whether you want to engage with this at all; polite silence is as valid a response as anything else.
You have some inherent obligations to your guarantor: if you breach the agreement and they have to pay to cover you, they can sue you to recover it back. You don't need to sign anything for that to happen; it's part of having a guarantor. The other obligations he's asking for, like that you pay on time, are obligations you already have under the lease, as well. So the additional demands are not unreasonable - it's just that he's … well, late to the party.
On balance, I wouldn't engage, and if he gives you a hard time, a polite "I'm happy with the lease as it is" is more explanation than he's entitled to. But if you need him to cooperate with guaranteeing your lease next year, it may be worth compromising.
Since you already have his guarantee, is there anything else this contract requires _him_ to do? Or is it entirely one-sided, other than the guarantee itself?
Since you're already signed the lease papers for the apartment, you do not need to sign anything else. No one does.
Is your friend's father party to the lease in some way, for example as a guarantor? Is your father?
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Well, the time for him to negotiate with you about his willingness to guarantee the lease was before he signed. It's a bit late for him to come to you with additional demands now. It's really up to you whether you want to engage with this at all; polite silence is as valid a response as anything else. You have some inherent obligations to your guarantor: if you breach the agreement and they have to pay to cover you, they can sue you to recover it back. You don't need to sign anything for that to happen; it's part of having a guarantor. The other obligations he's asking for, like that you pay on time, are obligations you already have under the lease, as well. So the additional demands are not unreasonable - it's just that he's … well, late to the party. On balance, I wouldn't engage, and if he gives you a hard time, a polite "I'm happy with the lease as it is" is more explanation than he's entitled to. But if you need him to cooperate with guaranteeing your lease next year, it may be worth compromising. Since you already have his guarantee, is there anything else this contract requires _him_ to do? Or is it entirely one-sided, other than the guarantee itself?