Answer: It is too late in the season to fertilize as this would encourage late season growth that will not have time to harden off before the cold of winter and consequently can cause excessive winter damage. You could fertilize normally next spring; do not be tempted to overfertilize as this would not be helpful.
At this point I would suggest a wait and see approach.
The new growth at the bottom should fill in again over time. You could pinch out the new growth tips a few times as they come back, you would do this to encourage branching at the base to help them fill with dense rather than leggy growth.
Another option would be to cut the entire plant back very hard in late winter. This type of severe renewal pruning would put the overall plant into balance as it regrows. It would regrow fairly fast due to the established root system, but it does look terrible for a year or two. If you did this, you would also have no blooms next year.
Another option would be to selectively thin the remaining upper branches a bit to help the plant look more balanced. This would be done in the spring right after it blooms. If you go this direction, keep in mind you would not want to remove more than 25 percent of the branches altogether, meaning what was cut in error plus what you would be cutting to thin.
I hope this helps.
Q&A Library Searching Tips