In favour of this reading we could immediately suggest:
a) it has outstanding early manuscript support;The main difficulty with AUTOU seems to be internal. Hebrews introduces the citation as God's address to the son (1.8), and this fits the pronouns, including three definite occurrences of SOU and one SE. The AUTOU reading interrupts this pattern, whereas the alternative reading SOU maintains the pattern ('your throne ... your kingdom ... your God ... your fellows'). AUTOU disrupts the pattern in two ways: it introduces a third person pronoun without any clear antecedent (whose kingdom is 'his kingdom' in this context?); and secondly the only logical answer is that AUTOU refers to God, but since the whole citation is presented as a declaration of God directed to the Son, a reference to 'his kingdom' doesn't really fit.
b) it is unassimilated to any known LXX reading (hence transcriptional probability suggests the movement to harmonise the reading to the LXX); and
c) it is the harder reading (since it is not clear to whom the pronoun refers).
So I would prefer to read SOU here, but I think perhaps I should actually read AUTOU. What do you think?
Here is P46 (just for interest):