Traditionally in English -- as in many other languages -- when you don't know the gender of a person, or you are referring to some generic person who could be either gender, you say "he" (or whatever form is appropriate). "I forget his name."
Some today consider this sexist and demeaning to women, and so use other words.
There is the practical problem that it can be unclear whether you mean that the person was or must be male, or that you are leaving the gender unspecified.
No, you cannot use "it". "It" refers to inanimate objects, not people. It would be confusing at best and insulting at worst.
Some use the "singular they". That is, they write, "... I forget their name". The catch to this is that, just as using "he" can create ambiguity whether you mean "male" or "unspecified", so "they" can create ambiguity whether you mean singular or plural. For example, if I said, "Every mechanic is required to clean his tools after work", clearly I mean that each mechanic is responsible for his own tools. But if I say, "The mechanics are required to clean their tools after work", now it is not clear whether I mean that each mechanic is responsible for his (or her) own tools, or whether the mechanics as a group are responsible for all the tools.
Some say "he/she" to make clear that they mean either gender. "I forget his/her name." Possible, but it gets awkward fast. "He/she must write his/her name on his/her form, and then give it to the clerk who will submit it to his/her supervisor" etc.
Some have tried to invent new pronouns which can refer to someone of either gender. Like "xi/xem/xis" like one would use "he/him/his". The problem with this is that inventing a new word and getting people to use it is hard. It can work for the name of a new invention or a technical term. Like when the cell phone was invented, there was no name for such a device, so someone had to invent one, and "cell phone" and "mobile phone" seem to have caught on. But we already have pronouns. And to make it worse, we use pronouns all the time and in many contexts, heck, in almost EVERY context. You often use pronouns multiple times in one sentence. Reading a paragraph with newly-invented pronouns, they just jump out at the reader and are very distracting. Maybe, possibly, advocates of these new pronouns will succeed and at some point get "over the hump" so that readers see them as normal. But we are a long way from that now.