You shouldn't be in that situation. Antibiotics are not supposed to be discretionary medicine that you take until you feel better. Your son should have finished the prescription. Stopping antibiotics prematurely is one of the ways that antibiotic resistant bacteria develop.

Source: I prescribe antibiotics

This situation is legitimate.

Example: I prescribe you 5 days of trimethoprim for a urinary tract infection. It's 1 tablet per day. It comes in a box of 7. You can do the maths.

Edit: and as others have suggested, if patients ask me what to do with extra meds of any kind I suggest they take them to the pharmacy.

He had an allergic reaction and we had to switch drugs after one dose. But thanks for the unsolicited advice. ;)

Why not bring it back to pharmacy? In UK and other EU countries pharmacies collect and have ways to dispose of medical waste. Same goes for the shops that sell alkaline batteries - they have special waste bins for disposing of used ones.

Mixing it up with excess bleach will certainly eat up all the active ingredients within a few hours. Best done outside in fresh air

I can't tell if you're serious, but it almost sounds plausible.

I'm completely serious, my undergrad degree is in chem and my grad degree is pharmacology. Bleach effectively "burns up" most organic chemicals. Bleach is so reactive that, while it's "dangerous", it's not a persistent environmental toxin. The thing about ventilation is re:reactions with amines (most drugs have nitrogen's) that make tear gas-like stuff. Whomever disposes of antibiotics is doing this, or chucking it in an incinerator. Burning it is better, but e.g. an oil drum filled with garbage and some gasoline will vaporize a fair bit of stuff and complete combustion is not nearly convenient as stuff you can buy at the grocery store

The antibiotic series I took this winter all had big warnings (presumably from more recent CDC guidance?) that extras should be disposed of in their sealed containers in refuse/trash and never flushed or poured down a drain.

In the UK one is requested to return unused medicine to the pharmacist.

I’ve never actually done this

In my experience this is trivial to do; just hand it in at the pharmacy counter. Never had an issue with it.