it's just an age thing - they do get over the losing strop, honest. (the more they lose, oddly enough
)
ds1 just played against his sisters at that age (one older, one younger) but it was only him that threw a strop if he lost. if it helps, he was exactly the same at monopoly and all other board games at 5. he's much better at 9 - can lose at chess without an end of world scenario developing!
at 5, dd2 just got bored and didn't want to play any more if she lost. as the youngest of three i was more worried about that than the losing strops of ds1 tbh. and if dd1 lost (as the eldest) she cried for an hour.
on balance, i think the strops might be easiest to ignore! 
oh, just realised you want to cure it by getting him to win every time instead 
ignore me... nothing to see here.
although - being able to lose magnanimously is a good skill too, y'know.
dangers of perfectionism and helicopter parenting on the horizon?