There is little to be gained from the photo - his branch badge, showing his trade/job is on his right arm, but cannot be seen clearly in the photo. He is wearing a single medal, which may be the Long Service & Good Conduct Medal [blue with white edges] - problem with that is that it was introduced in 1848, and is still being issued, so there are thousands of recipients. I am not aware of one source that would help with that, but perhaps one of the genealogical sites may help? - see also the Wiki page on the Naval Long Service and Good Conduct Medal (1848)
As to the date - I think this is almost certainly pre-war, probably mid/late 1930s. He has three good conduct badges - each one represents four years service, so he has done a minimum of 12 years time - plus he has what could be a LS&GC medal, which is awarded after 15 years service. If this was taken in the 1950s, he would certainly have wartime service medals - which he does not. The photo would fit someone who joined the Royal Navy after WW1 but before WW2.
Your advice on the cap was not strictly correct. Before the early 1950s ratings had two hats - white for summer and black for winter, although during WW2 only black caps were worn. So the photo could well date from the 1930s, taken during the summer, which seems to be correct, as he is sitting in a deckchair...
