
Just a little joke, but only dogs that haven't been castrated
can get testicular cancer - you can't get cancer in something you haven't got! ;-) :-D
That aside, the chances of an entire dog getting testicular cancer are quite small - about the same as for a man, apparently. Yes, it happens, and people are advise to check routinely, but it's not common.
Is there any other reason for castration? What's his personality like? At least if you do decide to castrate then you've left it until he's fully mature; dalmatians which are castrated too young are more likely to suffer urinary blockage from stones, but you won't be increasing his chances of that happening.
If it helps, my dalmatian boys are 8½ years and entire, and have had no problems (so far!) from being entire.