What I have seen here is two factors that lead to masayoshi son lose in bitcoin, first he followed the advice of a bitcoin market booster which is very wrong because in a volatile market such as bitcoin there is no professional analyst that can give 100% prediction on the future of bitcoin market and secondly haven invested in middle of 2017 when the price was heading to the sky he should have hold his bitcoin when is has gone down and already running at lost. Because you only lose when you sale below cost price so in his best interest he should have considered hodling till the market recover before selling.
Son has very good experience on investment, I do not think that his decision to buy bitcoin was because he followed a professional analyst or expert or anything. Anyway, the amount lost is not that big for him imho, as his SoftBank has total asset value estimated $190 Billion. Indeed lose is still a lose but I do believe he sold it at that position with a reason, although we do not know what the reason is.