He is pretty stoical but he cries out sometimes when he is bad.
Then suddenly he seems better. I have rather avoided the vet in case it is very bad news.
Many people do the same and end up like you worrying unnecessarily and failing to provide decent pain relief for their dogs!
It very much sounds like your young chap has a condition called Panosteitis.
It is not uncommon in growing large breed dogs, especially males, and symptoms usually involve severe pain in the long bones, with the humerus in the foreleg being affected most often.
As you have seen, lameness is intermittent and starts around five months of age but it can continue until dogs are well over a year old.
Fortunately, unlike many of the other growing disorders of bone, no long term damage is done.
We are not sure of the cause, with genetics, high protein diet, over exercise, rapid growth and plain bad luck being involved.
Diagnosis relies on palpation of the long bones during an episode (but watch out! Some of these dogs are so sore they will bite you!) and X-rays will confirm the presence of abnormal fluffy densities in the medulla of the bone.
Treatment involves rest when times are bad and decent pain relief. So get your poor young man to the vet!