He is worried by something and rather than dismiss it or try to make him feel silly ("we won't buy birthday presents") I think you should help him by trying to find little positive things. Suggestions:
let's always take a shopping bag so we're not buying plastic (can you help me remember? it will help the animals not to get trapped in litter and it will help the air as well because they don't have to keep making more plastic bags)
could we join a litter picking group or do a regular day of litter picking around your local neighbourhood on your own? (can you and I do this together? it will help the animals not to get hurt)
could we walk to the shops, at least sometimes (can you help me carry the bags? remember if you can't do something every time doing it sometimes will still help)
could we stop buying plastic bottles but always take our own mugs if we go out (could you help me remember?)
can we do days out without the car? (let's make it a challenge, get the map out and you can help me plan)
could we plant a bush that butterflies and insects like? (explain how this could also help the birds and the bats)
cook vegetarian/vegan food once a week/however many times you think is sustainable (can you help me think of things to cook? can we find out more about nice things that are good for the environment?)
could we find out from our local RSPB or wildlife organisation what we could do to help the birds or animals? (something like bird watching or insect counting can sometimes help them plan, maybe nesting boxes)
could we find somewhere to go on holiday (if you can afford holidays) that we don't need to fly for? (can you help me find out?)
could we plan how to get around with using the car as little as possible (let's make it a challenge and find out all about it!)
can you remind me to switch the engine off? (annoying as this may seem, it does help, and you will find it less annoying if he is doing it to please you)
could we stop drinking fizzy drinks and drink water from the tap?
could we feed the birds or plant a bush they could eat from?
could we cut down on cleaning stuff and use things that aren't so harmful? (you could help me clean)
could we sort out old things we don't need and take them to the charity shop so other people don't have to buy new things?