We always ask parents to come on trips because of safety issues with roads and transport and most parents quite like to come with their LO's. However if parents really cant come staff are able to look after children at the venue but we never put children in our cars.
So I would recommend you ask the staff if someone can look after him and then either you drop him off and pick him up or ask another parent to take him.
It will be ratios however it should not mean that a child misses out because their parents can't go. They can ask for volunteers of course but should have people in place before announcing/setting the trip in stone.
Ask the staff, I couldn't make a couple of trips either and the school said they were happy to cover. It's just that they do need the majority of parents to help otherwise they won't have enough helpers.
Pre school has a theatre trip this term. We were asked if we wanted to go along to help, but it isn't essential. Are you sure you've read the message correctly? I've just posted a message asking what a letter from school means!