I know the passage. In fact, I printed all the installments in AJ and then sought out some of the Japanese originals, as preparation for TIE 17. I think the only way to throw any more light on Sokatsu's education is to search through Aizu records. Since Sokichi taught Sokaku, his second son, it would seem highly reasonable that he followed Tokugawa tradition and taught his oldest son, too, and probably with greater care, because he was the oldest. In addition, there is an issue of whether Sokatsu was high enough in rank to be eligible for attending the Nishinkan, even though Bange was a long way away.
Peter - further meetings with his father is a very important question, of which I've no answer. And Sokichi died in, I believe in 1904. Those last years are very intriguing.
Here's the best info
I've read in English regarding Sokaku's early behavior, his training directly from his father, etc. The article notes Sokichi's education, older brother Sokatsu's martial abilities (where did HE get them), and the repeated social distress that Sokaku "caused" his father.
One passage is particularly worth quoting and one sentence there is highlighted: