The phrase "our boys ended second" should be acceptable, although "our boys ended up second" is apparently ok.
I suspect it's important to understand the exact meaning here--which I don't. Is second meant as in second place (such as in a race), or that the boys were the second team to complete some task?
Both are possible on the Czech side. I have reported "Our boys ended second!" (Only the exclamation mark is a bit more suitable/expectable for the first meaning - .... finished second).
Note: "Our boys ended up second!" is accepted.
why not 'the second'? is it a cind of an idiom?
You use "the second" when a noun follows, "the second place", "the second beer" or something else follows "the second to know that...".