Well, if you took nothing but cleric levels froim this point forward (assuming you do get approved to do so), you'd end up with 17 levels of cleric, which is enough (just barely) to have access to ninth level cleric spells. However, you would of course need a 19 wisdom score to actually be able to cast the 9th level spells.
Yes, by 40th he'd have a 15 just from normal stat increases every four levels. The Great Wisdom feats do increase "base score," and therefore should allow for further spellcasting, meaning if he uses four of his remaining feats to take Great Wisdom, he'll have the minum needed to cast 9th level spells by 40th level. He wouldn't be able to cast ninth level cleric spells until 40th level anyway, as that's when he'll become a 17th level cleric.
You should also get a feat at 24th level, so if you increase the ability score and take the feat Great Wisdom at 24th, he'll have a 12 Wisdom, allowing him to cast up to second level spells (which he will gain upon reaching third level cleric, or 27th level, at which point you can take another feat to increase his Wisdom again).
