This is a hard one. Normally, a sleeve is lengthened before beginning the armhole shaping. That is, for 'set in' sleeves. For raglan sleeves, it could be just the same.
How tall is this man? Just average 6' tall? Or, 6'4"? 6'8"?
If he's a 6'4" or taller man...he will not be an average armpit-to-base-of-neck length either.
Remember this, the number of rows you add to the length of the raglan region of the sleeve must also be added to the raglan region of the body, too. Both FRONTS and BACK.
Is this sweater a surprise? If not, why not ask him to measure himself from armpit to base of his neck? This will give you a fairly close idea how long he is along the raglan line.