Yes. Sentencing status has no bearing on whether InmateAid can reach him. Whether someone is pre-trial, awaiting sentencing, already sentenced, or mid-transfer, the letter service works the same way. InmateAid routes mail through the US Postal Service to wherever he is currently held, and pretrial and pre-sentence detainees in county and detention facilities receive mail the same way sentenced inmates do.
Send the letter, include his name and inmate ID number in the addressing details, and it will go out. Mail is one of the few constants that does not change regardless of where someone is in the legal process.