Animal companions with an intelligence < 3 have a rather small list of feats to choose from, as stated here. Given the fact that your T-Rex is strength based, then the feats that come to mind are Power Attack and Improved Natural Attack.
At level 7, Power Attack would let your T-Rex take a -2 penalty on attack rolls to gain a +6 bonus on damage rolls. (I would rule a +8, because of Powerful Bite, but since this is PFS, it's best to settle for Rules As Written.)
This penalty can be turned on or off, so if you find you're having a particularly hard time scoring a hit, you can turn off Power Attack to deal normal damage without the attack penalty.
It is important to note that the bonuses from power attack scale as you level up, but they shouldn't change until you reach level 13.
At level 7, improved natural attack would permanently bump up your T-Rex's bite damage dice to a staggering 3d6. This is a passive ability that has no drawbacks, so if you have to pick just one feat, I would recommend Improved Natural Attack over Power Attack. (Unless you've been eyeing bull rush or overrun combat maneuvers, because Power Attack is needed to qualify for those feats.)
You can read the particulars about these feats here:
Improved Natural Attack
Sorry, I didn't see that your T-Rex's Intelligence is 3, I dug a bit and it seemed that most of the good feats required way more than 3 feats to build for, but I found a few that could also work for your T-Rex:
Blood Feast, A +1 attack and damage after your first hit with your bite attack.
Cleave, You can gobble up 2 foes at once by taking a -2 to AC. Requires Power Attack.
Great Cleave, Only useful for mowing down the mooks, you can use cleave on as many enemies you can reach. Requires Cleave and Power attack.
Furious Focus, You no longer take the penalties from Power Attack on the first attack you make every turn! Requires Power Attack.
Improved Sunder, You can chew up your foes weapons and armor, leaving them squishy and vulnerable. Requires Power Attack.
I would recommend Furious Focus, unless you want to avoid Power Attack, then the only other damage feat I could find was Blood Feast. It is important to note that your T-Rex can't take Vital Strike until level 9.