Hello Peter,
Firstly, I can see nothing on the CWGC site for a J. V. Parry MN. There are some J. Parrys but no Cadets. So this would suggest that he is not mentioned on the Towerhill Memorial as being lost at sea from a war cause. If he was lost due to a weather related incident or a collision at sea then unfortunately he would not be commemorated on Towerhill which is a disgrace really.
I suggest the way to go here would be to attempt to get hold of his CRS10 (service record) which should be held at the National Archives in piece details BT382/1383 at this link here:
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If he is filed under (Vincent) Parry then this link here:
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Now unfortunately, as you may be aware, the National Archives have had problems with unauthorised access to personal details and as a result have stopped direct access via the Digital Express service to Seamen's Pouches and CRS10s. This is still ongoing and we do not know when or if full service will be resumed. But If you hit the first link above you can request an estimate for a pdf online copy. This may take a few weeks to complete though.
I take it you dont have a date of death?
There are other ways to find out but the lack of details makes this very difficult and the CRS10, if it has survived, would be best.
In the meantime I will keep digging, see if I can come up with anything.
Regards
Hugh