Maybe you could try Pure Neatsfoot Oil? Read the label--Pure Neatsfoot Oil will tend to say something like 100% natural pure; Neatsfoot Oil Compound
has added ingredients like petroleum or synthetic oils. It is a relatively pure oil rendered from cattle leg bones. It will definitely darken the leather though, so it you're attached to its color, you might not want to try it. But I dunno.
And just to make it more difficult to figure out what is going on, if he does seem to have a reaction to the collar, he could be reacting to the chemicals used to tan the leather, not necessarily to anything you put on it.
I find dobes a bit difficult to fit into a collar. Their heads tend to be about as big around as their necks
You should be able to fit just a finger or two under his collar when it is on. Which to me seems like I have to buckle it on, not just try to slip it over his head.
If you use a choke collar for training (which a lot of people don't--it's not necessarily the best collar to train a dobe with), they even make a toggle collar that you don't have to slide over his head. That way you can get the collar to stay right up behind his ears. If a collar will slide over his head, it will tend to slide down his neck too.
Weight--75 pounds is fine, and at his age he's likely to gain a few more pounds. He's not quite a mature dog yet.
Whether a dog is a good weight depends on his size and bone structure--dobes are supposed to be medium sized dogs. At a proper weight, you should be able to see his last few ribs and feel the rest fairly easily with no apparent fat padding. He should have a definite waist when looked at from above and a tuck up (tummy line) when you look from the side. He shouldn't have fat rolls at the base of his neck or in front of his tail; vice versa, his hip bones and backbone shouldn't be sticking out.
Anyway, if he's fit and trim, it's not particularly useful to worry about his size and weight. He will be what he will be.