I think, at the risk of disappointing people who are anxious for Rush to endorse, he is 1) not willing to subvert the primary process the way Levin and Beck have chosen to do, 2) trying to appear neutral so as not to divide the base, and 3) most importantly, trying to coach Donald along by making him aware of some points he might need to firm-up in his conservative credentials (of which Rush perceives Trump just is not that ideological, more pragmatic -- but that he needs to become more ideologically astute).
You will find Rush firmly behind the Repub nominee, equally all-in as you and I both are. Compare, for instance, Rush's coaching, vs Levin's strident, truculent demands.