I never tried, but there is a romance route (just checked via debug. It's there and actually quite complete).
On the second dose, make him less abrasive instead of having him come to the lab. Then convince him to be your best friend when the next branch comes up and you can end it with full devotion.
>9000 hours in mspaint later and I made a tree of John's routes (excluding his dom route, just suck his dick before dosing him and it's begun without opportunity to change course):
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