My brother in law is a rich man and can have anything he wants. Is there anything unique you can recommend me giving him for his birthday?

Since he is rich, what really can impress his must be unique and special rather than just expensive. I would recommend the PERSONALISED KEYRINGS made of sterling silver. What make this personalized keyring so special is that the entire personalized message (or your brother in law's name) is CUT OUT from the sterling silver plate (not normal engraving).
These unique personalized keyrings(a few different designs) are exclusively designer and available at UnusualDesign

Currently only Unusualdesign is making and selling these unique and yet prestige personalized keyrings

Vestal Men's Chronograph Gold-tone Purple Dial Watch is a nice watch.  It is a retro style watche.  It has a solid mineral crystal.  It has 3 eye chronograph movement.  It is made of stainless steel with a butterly safety clasp.  It is also water resistant.
There is the Personalized Red Carbon Fiber Dog Tag.  This makes a great gift because men love these and you can have it personalized with a special saying for your brother in law.  It has a gunmetal finish.  It gets high reviews. 
You should look into the multi-tool.  This is anodized and has several functions.  Men love these.  They are great when he needs tools on hand.  He can carry them in his pocket and it comes in several colors.  
I also recommended speakers and a ring too.  I hope this helps you in your search.  Thank you for the question.