Any ideas on how to make this tree bear fruit?

I'm thinking you should leave it to do it's own thing. This tree definately
qualifies as hardy but I think it's still recovering. I suspect it's trying
to build up it's strength again and may need a while to do that. Has it
flowered at all yet??

I have a mandarin that (before I knew better) I planted out in a root bound
state without loosening the roots first ... I know a lot of gardeners who
would have cut it down long ago, but I ignored it (I'm a big softy about
plants) and after 7 years the bloody thing began to grow! It's now looking
like a normal (young) tree and it's up to about 40 fruit this year (3 years
after it started to grow).

But that's just me. If a plant is going to die it does it by itself here. If
a plant seems to be not doing well I look after it, mulch, normal amount of
fertiliser and water, bug and disease control, and see if that helps it
recover. Then I ignore it!