They make slow release tree fertilizer spikes. Try digging a few in as deep as you can get them, around the drip line of the tree, NOW, so that by bloom time, it will have had the benefit of them.
If it does bloom next spring, be sure as soon as the blooms are done, before they set seed, you deadhead (clip all the spent flowers off). Lilacs if allowed to set seed, tend to bloom only every other year. It's an evolutionary mechanism to give the new seedlings a chance to get a little established before more competition comes along. Since the lilac seeds are in clusters and are heavy so don't tend to travel very far, the tree has developed this mechanism to allow the seedlings to develop.