The data I have are for the older E32 models (88-94), but I suspect the same system was still being used on your E38 (95-01).
The problem may be the fluid that your mechanic put in. The "iL" cars have a self-leveling suspension that shares a fluid reservoir with the power steering system. The proper fluid is Pentosin CHF (?)-the exact type is written on the top of the reservoir cap. The only source I know for this fluid is the dealer and it's fairly expensive, around $20/liter. OTHER FLUIDS WILL DAMAGE THE SEALS AND CAUSE LEAKS IN BOTH THE POWER STEERING SYSTEM AND THE SELF-LEVELING SYSTEM.
You'll have to have your hoses repaired of course. If the mechanic put in the wrong fluid you'll need to have the entire power steering/self-leveling suspension fluid flushed out and filled with the proper fluid. (There's a special procedure - the BMW way.)
Be warned, the self-leveling rear struts for this system are very expensive. The cheapest price I've found on the internet for them is around $400 each, yes each. Then there's labor for installation and the expensive fluid. If the fluid is wrong and the struts are leaking they'll have to be replaced.
If the mechanic used the wrong fluid I'd have the repairs done properly and send him the bill, which will be in the hundreds of dollars and well over $1000 if suspension components are damaged. His choice would be to pay the bill (not likely) or meet me in small claims court. After all, he's supposed be a professional.
If he did use the proper fluid, well, bad luck I guess.