Currently, on 90mg Prozac for OCD & depression. Doctor is suggesting 100 or 120mg.

Regarding the dose increase of the antidepresssant, I believe there will be changes/benefits and most likely a possible increase of more of the common side effects, hopefully not.
Speaking from experience I started taking medications in my 20s ( I am almost 56 now, male) and everytime there has been a significant dose increase there have been benefits and common side effects in my case, but the side effects did subside within a week to 10 days, now everyone is different and medications work differently for everyone.
I did take Prozac (Fluoxetine) about 30 years ago but my dose was lower I can not remember how much but Prozac is a very good SSRIs (drug classs- selective serotonin reuptake inhibitor).

With regards to Abilify (Aripiprazole), for me it was a very good add on choice, as this is a 2nd generation atypical antipshychotic meaning you can expect less side effects than the first generation antipsychotics.
The max dose I believe is 15 mgs., I was on 21.5 mgs for about 6 months and I did tolerate the med quite well and the benefit was greater than any potential risk involved, again , a very good medication, and also everyone reacts differently.

Of course there are apprehensions when starting a new medication, my advice to your teen is not to get anxious when sgtarting Abilify and not be afraid of possible side effects he may or may not experience, remember they are potential side effects.

Here is an interaction report and information on Abilify, just click on the links:

https://www.drugs.com/interactions-check.php?drug_list=233-109,1115-648

https://www.drugs.com/abilify.html

There is a moderate interaction between the meds, moderate meaning:

Moderate Moderately clinically significant. Usually avoid combinations; use it only under special circumstances.

His/her Dr. most likely is aware of this potential interaction and has outweighed the benefits are greater than the potential risks involved and would not put his/her patient in harms way.
I do suggest you dicuss the interaction report with the medical professional of your teen for your peace of mind.