Yes, Japan has a very low (below replacement level) birth rate, but there is nothing to indicated that the Prince would not be able to produce children with the right woman and if enough children, one may be a male to extend the royal line.
He just needs the right matchmaker and a willing partner interested in having male heirs. That should not be too difficult to arrange.
He seems like he’s a bit light in the loafers. Japanese men may not be hulking he man types but they tend to be comfortable being men.
There are literally no males for three or four generational lines out and they are OLD. No j re males anywhere out there. Not first second or third cousins if I remember correctly. If you marry into the Royal house you must cut off ALL ties with your family and never see them again. Occasional phone calls are allowed.
I taught English to the aunt of the Prince’s mother. Interesting lady. She was allowed to attend the royal wedding and that was it.
I think that’s why the current emperor’s father married a commoner and each of his sons did as well. Too much inbreeding. Also too much pressure on the wives. Nobody wants to marry them. I understand the isolation and criticism are extreme.