British Airways had nothing to do with this; it was Aviapartner staff that denied you boarding.
Your son cannot use his residence permit to transit the UK (only the common EU-format permit card or the family member card – which looks like the Belgian national ID card – is accepted), but the US visa would’ve worked.
Unfortunately, it would appear there is a bug in TIMATIC, the database used by check-in staff: when Nationality=India and Transit Point=United Kingdom, it says:
Visa required, except for Nationals of India with a normal passport transiting through London: Gatwick (LGW), Heathrow (LHR) or Manchester (MAN) with a confirmed onward ticket for a flight to a third country on the same calendar day. They must:
have a visa issued by Australia, Canada, New Zealand or USA,
andstay in the international transit area of the airport, and
have documents required for the next destination.
However, when Alien resident of=Belgium, this doesn’t appear, and as such the staff denied you boarding.
Unfortunately you’re likely out of luck, as TIMATIC is run by IATA and is the “bible of airlines”. Nonetheless, you could try claiming compensation from Aviapartner and British Airways per EC261, by pointing to the GOV.UK website as well as the fact that, if not entering AR=BE in TIMATIC, the exemption for US visa holders will appear.
If granted compensation, you should be given €600 per person plus the unused ticket refunded.
I will also alert IATA of this likely bug, so the database can be corrected. I will let you know when this is done.
Credit:stackoverflow.com‘
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