I suspect it to be due to infection in the donor site where the grant is taken.
As he have the draining of pus and is also odorous, it should be concerned and taken care of.
It may spread deep and infecting the bone further and ending in osteomyelitis
I suggest you to do the culture of the draining fluid to find the organism and antibiotic sensitivity for the organism.
In addition please do complete blood count
check, to know whether he have any increased count of white blood cells.
Antibiotic sensitivity is essential to prescribe the proper antibiotic for concerned infecting bacteria.
Hope this helps.
Please write back for further queries.
Wishing you good health.