1. You can't do it without him being involved in some way, unless you get a Court to agree.
2. If you don't know where he is you need to pursue whatever means you can to try & find him, & keep evidence of everything you do - so you can produce it in Court later if you still can't trace him. You may need to employ tracing agents.
3. If you do find him & he objects to the sale then you can only proceed if you get a Court order - which you may be able to do if the owners of two thirds wish to sell.
4. If you can't find him, it is perhaps possible a Court would order a sale, but with his share of the proceeds help in a secure account for him.
5. All this is very likely to involve you in a lot of costs!