Re: X-13 Flotilla
Wow. The 13th submarine flotilla was not actually a front line flotilla, but strictly a training one.
It was not formed until mid 1944 as the XIII training flotilla comprising the 6 X craft XT1-6 inclusive.
The XT craft were based on the preceeding X craft, but with much gear removed and a crew of 3.
All were built by Vickers as yard no 927 and were distributed around the British Isles.
They were short lived being broken up after hostilities from the end of WW2.
So, in short, as you surmise, he may well have only joined 1 of these boats to be trained, a few weeks at most, but if he ever then became crew is doubtful from your next statement. There are a number of resources where you can get his full naval records.
The front line X craft and their associated chariots etc were the 12th or XII flotilla.
These men and their craft saw much action Worldwide with its attendant losses and valour.