If he is only 60 pounds at 5 months, I would guess he will be small for a Great Dane. I have seen questions on how big a puppy will get several times lately. There was quite a discussion on it on another site I am active on. No 2 people seem to be reporting the same results. I have almost concluded just as large breeds grow longer than smaller breeds, so larger members of a breed may grow longer than smaller members of the same breed.
One thing I do know is that puppy chow is formulated to produce fast growth. Even the large breed puppy chow is rich enough to promote fast growth at the expense of sturdy joints. The solution is an early switch to adult chow, phasing it in over about a week. Best to stick the adult version of the puppy chow you are feeding. I would make the next bag I bought adult chow.
Overweight is bad for a growing puppy's joints too. I don't think that is a problem with most Great Danes. They may insist on showing a little rib. Here is a great guide to evaluating his weight,
http://www.puppychow.com/products/po...condition.aspx If he is a little on the thin side, Great Danes are that way.