I think I blew it with my serviceberry

I planted a serviceberry tree in late Spring and a month later it was severely damaged by apple cedar rust. I was waiting for a return call or email from the nursery that I purchased it from. In the interim, I started reading...and reading online. Some sites suggested pruning all the branches that had infected leaves because in Spring of next year, the cankers would encircle the branches and kill the tree. In my case, that included every single leaf on the tree. So I pruned back all the branches. Now it's a leafless bunch of sticks around 3 ft tall. Well, I finally got in touch with the nursery and he said to just leave the tree alone and let the leaves get inflected and fall as they may. Clean them up, of course, and next Spring, treat with a fungicide. Ugh - too late - it's pruned. He said if I pruned it, the new leaves with take energy that the tree needs to survive winter and it would die over the winter. What have I done? Did I do the right thing? What should I do now??? Thanks for any help

I would say NOOOOOO, you never prune a young or new tree to the extent you have.
Tree's / shrubs need the branches and leaf to act tile drinking straws, these are the only way a tree / shrub can draw water ,feeds etc UP into all the top growth, allowing the branches to grow taller, wider and greener, then fruit or flower depending on type.

Also when you buy a tree / shrub from a grower, they actually know their stuff and knowledge to have pruned, shaped and cared for the plant to get ready for the sale point, and beyond, I'm talking about good honest growers who DONT want their customers to fail and NOT come back or recommend others.

Make sure the leaf drop is gathered up and BURNT as some diseases can hibernate in the soil and bounce back again the following spring ready for new leaf buds to break.

It's a pity the Nursery had no help line and able to answer your questions the same day however, on saying that, this is a busy time for nursery growers and garden stores BUT the guy was right, leave the tree alone, there is NOTHING ever to be gained by cutting down branches immediately, it takes several months / weeks even for any disease to show up and it takes same, or longer to rectify a problem, their is NEVAR an instant cure so you should never act instantly UNLESS your absolutely sure that's what is required, another couple of days or a week later would not have killed the tree either, UNDER the soil the real workings of a tree is happening and if it's still taking up water, making new leaf (EVEN IF DAMAGED) the tree / shrub is still working to stay alive.
By pruning so severely, you may be able to save the plant, maybe it will be hell to look at for several years but so long as it gets some new growth, it is still alive, so lot's of TLC is required, Allow the tree to recover, as stated by nursery, wait till next spring (EARLY SPRING) and then spray for the problem you have had, leave it to grow on for a few years more before you try to reshape the tree to how it should look for good fruiting results.
Hope you and the tree soon recover.
Best Regards.
WeeNel.