Hi there, For a start - have you taught him what 'off' means? So many people seem to assume that dogs understand English (or human language) because they do what we ask, when infact they have learnt OR are reading our body language and are making an educated guess. Dogs themselves communicate between each other using body language, but we are limited in this respect because we don't have the same physical construction, so, we have to teach them initially using a combination of body language and verbal commands.
To teach an 'off' command, I would use a small but tasty food treat (can be human food if dog treats are not available, infact human food is often healthier - a small piece of cooked chicken for example) hold it between two finger tips, so the food is covered mostly by the fingers (be prepared to remove it quickly should the dog lunge for it ;)) and lure him down to the floor, and reward him immediatly with the treat when his feet are on the floor. Slowly you can add a command word, which coupled with your body position will tell him what you want him to do. My dogs are trained to both command words and hand signals using this method.
He's possibly growling because your attemtps so far have been confrontational and not consistent enough for him to learn what each thing means. When training children or animals, you must be consistend - how would you feel if your boss asked you do to do something by saying one thing one day, and another the next without explaining what each command meant?
It may be easier to train the children he comes into contact with most the appropriate way to deal with dogs - not disturbing them when they are asleep, stroking the chest and throat initially, and not the top of the head, don't hug them or put arms across their shouldar area, don't stare at them or maintain eye to eye contact.
He is a Spaniel, and Spaniels do spend most of their time with their noses to the ground - it's what they were breed to do. You can try and combat that by using small food treats again, and get his attention on you by rewarding him when he looks at you when on the lead - try it at home at first, so he understands what action gets the rewards.
If you're in education, you might find this site
http://www.clickandtreat.com/Newbies/newbies.html interesting, as the techniques can be applied to both dogs and children ;)
House training in a small apartment may be difficult if your intention is that he should 'hold it' untill you can get him downstairs and outside. But if you have a veranda, could you teach him to use that? Failing that - a cat litter tray in the bathroom?
HTH