How is he accessing the file? FileMaker Server?
Its can often be better to access a local client computer (or vm) via vpn so that the work on the database is all performed locally.
Unfortunately FMP is very susceptible to Latency.
Redesign the data table splitting it into two table. Table 1 is the fields he needs to enter. Table 2 is all the other fields.
Create a special layout with only fields from table 1.
After he submits the data have a script create a new record in Table two linked to table 1.
Set up a computer in your office that he can use ARD or Microsoft Remote Desktop to access FMP.
Create a file for him to use in his office that he enters data into and you can also access. Then from your side run a script to pull the data into the main file. The import can be set up on a 'Robot' checking for new data on a timed event.