Hi,
hehe, you do set TI in ser_init() via:

SCON = 0x72; // Mode 1, REN 1 and TI=1 as well!

I thought that your ser_putch() is same as often-and-often used next one:

{
while (!TI) ; // wait till prev. char has been sent
TI=0;
SBUF=ch;  // send next char
}

...but I was wrong. Ehm, how many programmers so how many program ways to do one and same task (even an error ;)
Check your C51 code. I think you clear TI somehow there before send the first char.
Good days!