So, from what I can tell, maybe these could stack.
The issue I'd hold is that in order to get this combo,
1. Your player must reach 3rd level, not 2nd. The 2nd level ability of Titan Mauler is to wield a two-handed weapon in one hand.
2. You have to hand waive the use of a +1 CR template (Ogrekin) on his character. Normally, I'd consider that worthy of putting that player 1 or 2 levels behind the rest of the party (so he'd be wielding a Gargantuan weapon at 5th level or so, which is almost balanced against gaining Vital Strike at 6th except...)
3. You are allowing a 3rd party race that doesn't seem particularly well-balanced: Fire Acclimated, Naturally Psionic, Half-Giant Psionics, and Survivor are each roughly equivalent to a feat, and Powerful Build is even more powerful still.
On the other hand, each of these abilities allows him to access a weapon "1 size category larger than normal", so I'd probably say what he really has is the 3 different abilities that say his medium character can now wield large weapons.
Ultimately, I'd say you're playing with a lot of GM-fiat and using 3rd-Party materials. It's ultimately your call as to what would be considered balanced. As far as what builds could be made when you take off the limitations on templates and 3rd party materials...
'Here, there be monsters.'
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"She can use two-handed weapons meant for creatures one size category larger, but the penalty for doing so is increased by 4."
"The ogrekin can wield weapons one size category larger than normal without any penalty and gains a +2 bonus to its Strength."
"A half-giant can use weapons designed for a creature one size larger without penalty."
The character's size never changes. At most they can get to Huge because the normal weapons a Half-Giant can use are Large.
Also, you need to point out the pricing, weight, and availability of a Huge Weapon.
Finally, he should have to roll for what the template gives him, not just pick what he wants.