I have 10 or 15 January dorsets out of an April born. He was settling ewes by the 15 of August. That being said, breed has something to do with it. Expecting a young ram to be fertile in July is asking a lot. Many mature rams are not fertile in July. If I was determined to get them bred in July,I would be using and proven or at least more mature ram. Any ram lamb,needs to be well fed,not excessively fat, and taken care of. Mine are pulled daily to feed by them selves. He bred ewes for Jan,Feb,March and April. I have April born hamp ram lambs, twins out of a 12 month old first timer. They both were good to go in late August .1 was used on 2 matures in early September. The other was not used until late October and November. Again well fed, kept in shade during day, fans on 24 hrs a day from weaning until turned out, and then had fans on them as well.
Size has something to do with it as well. Some of the " trendy" rams lambs,simply are not physically big enough to mount large mature ewes.