Over here there's shelf products available.
People can spray growth retardants or plant growth regulators on anything really, it's hormone based. Ask around for it, here you can by stuff that plugs on the garden hose and spray away.
Some hi-tech info
Using Plant Growth Regulators on Containerized Herbaceous Perennials
Some products here but do your own research.
Growth Regulators - Plant Growth Regulator, Plant Growth Retardant, Bonzi Plant Growth, A-Rest Plant
Root growth stimulants, I have even used rooting hormone powder you strike cuttings with ... but for the volume you need forget that.
Here's some leads for you on root growth stuff.
RGS Root Growth Stimulator Mycorrhizal Root Growth Enhancer
Ask the arborist if there's any injectables around too, perhaps a few stem injections of something suitable might give the tree a chance too.
Just remember, lay off the N fertilizer, N promotes fast growth and greening of the canopy and you need root growth. All products have some N in them, look for a low one.