Engine Knock

My uncle has a 96 GMC s-15 2wd 4.3l vortec, and has 199k miles, and it just recently
started making a terrible knocking noise. almost sounds as if something is loose, but,
heres the scary part, the other day he came home and pulled in the driveway and the
truck was idling rough and nearly died out on him, and steam was pouring from the
motor, i pop the hood and his brake master cylinder lid was loose and brake fluid was
everywhere, it knocks on startup, and when its warm. but its not a loud knock until you
give it gas. my uncle, bless his dumb lil heart, "SAID" he changed the oil and put
synthetic oil in it after it had regular 10w-30, im thinking he didnt change the oil, and
kept the same ooil in it and just threw synthetic in it, but would it make that noise? idk
im no mechanic, or could the brake fluid maybe have gotten down inside the cylinder or
whatever and is making that noise. his oil pressure lights reads a little high also and he
also stated that he has stalled out coming up the road, but would the engine knock
make it stall out? i feel bad for him bc now he has no vehicle, and id like to help him out
by figuring this out, hes ready to scrap it but i just want to make sure its not a simple
fix, or if he needs a whole new motor? please help and thanks in the advance.