Very confused about UK ETA regarding Indian citizen who has a US green card

Any UK lawyers or travel experts out there? Or if not, can you please direct me on who to contact? I've spent hours googling ...

My daughter and I are US citizens. My husband is a citizen of India and is legal permanent resident of the USA (green card holder). We booked a trip to India in May. Our return trip is British airways from Mumbai to LHR, 2 hour layover, LHR to DFW via British airways/American airlines. (Yes, this is a single ticket . )

I applied for and got the ETA for my daughter and I, but I am very confused as to what my husband needs in order to transit through the airport. Prior to the ETA law going into effect, neither US citizens nor US green card holders needed any kind of visa or travel document to transit through the UK. Now, US citizens need the ETA; so where does that leave the green card holder? A citizen of India would normally need a visa, but since husband has a green card, the law stated he would be exempt. Again, I'm very confused as to what category he falls under now. Who can I contact for clarification?