Unbelievable! Of course you can't sue, how ridiculous!
Toddlers have accidents all the time, that's why you need to adequately supervise them to try and minimise the risk. Note how I said minimise and not eradicate - this would be impossible!
Restaurants are busy places, full of potential hazards. The only place your DS should have been was strapped in a high chair or sat on your knee, or a designated children's play area.
As you've already said it wasn't a play area but a pub garden, then it stands to reason that there will be glasses out there, and the staff have no requirement to keep it toddler safe as its a space for adults to go to eat/drink/smoke, NOT for children to play in.
Children don't 'need space' in restaurants, they need to sit nicely and eat. If your DS isn't able to sit for the length of time needed to finish the meal, you need to either leave him at home, or go to a family friendly place that is geared up for kids, with a play area to run about in.
I get that your husband is American and therefore comes from a culture that sues as the drop of a bloody hat, but perhaps remind him that here in the UK we tend to be less grabby blame focused and can appreciate that ACCIDENTS, happen.
I also agree that the ambulance was massive overkill, a taxi would have been a better idea. Perhaps do that next time your DS is hurt, because there really will be a next time, this is what happens!! It's no wonder the NHS is on its knees 