Yep, definitely possible. HOWEVER, bitcoin might be worth nothing by 2020, replaced by some altcoin or something, so you might become a millionaire but you also might be penniless online.
No he need to wait long to become an millionaire as I don't think that price will be too high by the time of 2020 but if you hold 1000 btc then you can be millionaire for sure
Maybe the price will reach 1000$ by 2020 but no one really know may the price of bitcoin will just be 600-700 by 2020.