Change your sentence slightly (while keeping the tense) to make it more idiomatic.
A: I am not sure whether he arrived in London last night.
B: If he has (arrived), today will be perfect.
Use of the present perfect perfectly fits the situation.
B is assuming that if he arrived last night, he will still be there - and presumably they can meet up.
It would also be possible to respond:
B: If he did (arrive), today will be perfect.
using the auxiliary did with the base form of the verb arrive.
The difference is that conditional response if he did arrive does not assume that he is still there although this is still implied.