this is probably common sense but

[loosinas]

when my son comes along the sounds like 'ss' 'ff' etc would i be right in telling him that when he sees two letters the same they're pronounced just like the single one would be?

With "ss", yes, just a bit longer.
With "ff" not always; think of "of" and "off" - ie "f" can sometimes be said as "v".

Does that help? Remember that when he comes across things like "oo" and "ee" your idea doesn't work, in that a single "e" makes a very different sound to double "ee". But don't stress, he will cover the trickier sounds at school.