When opening the mouth, you can fold part of his side lip against the teeth so that if he tries to close it, he closes it on his own cheek.
You might also try a balling gun:
http://www.kvsupply.com/KVVet/productr.asp?pf_id=93620&gift=False&0=dept.asp%2Cdept_id%3D9000%26Tree%3D%2CComplete%20Catalog&1=dept.asp%2Cdept_id%3D50000%26menu_id%3D%26Tree%3D0%2CLivestock&2=dept.asp%2Cdept_id%3D51700%26menu_id%3D%26Tree%3D1%2CMiscellaneous&HSLB=False&mscssid=191F808CD54046549D86766AB630602D This tool can be used to position the pill at the back of the throat without risk to your own fingers and then eject it by means of a plunger, beyond the point of no return. There is a hump back at the back and if you get the pill over that hump, it will go down even with resistance from the dog.
Your vet may be willing to change the formulation of the antibiotic to a liquid. With some dogs, it is easier to give liquid medication. You tilt the dog's muzzle up and gently but firmly grasp the muzzle. Insert the syringe between the cheek and molars and slowly push the plunger. The liquid then trickles to the back of the throat and the dog must swallow. Liquid antibiotics also tend to be flavored and somewhat less objectionable to some dogs.
Some antibiotic tablets are rather large. For some dogs, it may actually be easier to break them in half and give each half one after the other, instead of the whole. Some dogs may find it easier to swallow that way.
You can hide pills in food, and I do that with things like vitamins and glucosmine, where there won't be a significant problem if one is missed every once in a while, but you must be very careful with antibiotics that the animal take all of the pills and take them all on schedule. Failure in either regard MIGHT actually give the bug a chance to regroup and come back stronger. So even if the dog seems better, you should finish giving the full course of treatment (lots of people will stop when the dog appears better, and this is a big no-no with antibiotics).
If you use food, monitor very closely to make sure the pill goes in, and not under the fridge. If the dog does sort it and refuse it, the pill may become somewhat dissolve on the outside, which will almost certainly make it harder for him to swallow as it will be more likely to catch in his throat. In such a case, I would throw away the damaged pill and use a fresh one, then replace it at the vet's the next day.
According to Cole, the surest way to a dog's stomach is still Fancy Feast. But poor Leon has those allergies and dietary restrictions, so I don't know what to suggest for him in that regard.
Now here's what I personally do with my own pill resister, Cole. I give the pill followed immediately by a highly desirable chaser. This has two positive effects. First, in order to swallow the treat that he's eager to get, he will also swallow the pill that went before it. Second, it's good training. I'm pairing something he's not thrilled with doing with a very positive reinforcer. I may even jackpot him with treats after he takes the pill.
Finally, gravity is your friend when pilling any animal. I find the ideal position with a large dog is straddling the dog as if I was going to ride a horse, with the dog on a sit. I tilt the head straight upward toward my face so I can see the hump at the back clearly, then I chuck the pull to the back of the throat and let gravity take it as far as it will go. In reality, with my dogs, I do just insert my hand in the mouth to the back of the throat because they are all used to it. I handle their mouths a lot and they don't object. My cats, on the other hand, have made vile threats of dismemberment during pilling, and for them I always employ the gravity method, except I rock them on their backs like a baby, their head cradled in my left elbow, my left hand grasping their front paws, and I open the mouth and drop the pill in with the right hand.
I have found with all animals that in the long run I get far less resistance if I develop a fast and efficient delivery technique. Then it's less traumatic and objectionable for them.
Re: biting the toothbrush during brushing. You can gently hold the mouth closed and insert the toothbrush between the teeth and cheek. Then he can't get the brush between his teeth to bite it. Dogs have carnasial teeth at the back, which means they overlap each other with the upper molars outside the lower molars. You only really need to brush the outside of those upper molars and the incisors (front teeth) and fangs. The incisors and fangs are easy to reach. It's those molars that are tricky. But you don't have to do the inside against the tongue, and you don't have to do the lower teeth. The one exception is that there is a grinding molar at the very very back. Just one on each side of each jaw (4 in all). Those should be brushed across the grinding surface if possible. But this is not always possible.