I would say, "He needs practice now." I don't think it sounds natural (though it's not strictly incorrect) to say, "He wants practice now." But maybe it's a dialect thing.
Generally I think people would say 'He wants to practice now' but it's a case of a translation being slightly unnatural in English but perfectly fine in Irish.
This could be used for it, yes. I think the use of ó in this situation really needs to be elaborated more on (personally, I say use teastaigh for "need" and ó for want...it's easier to keep them separate). But you could also say Tá sé ag iarraidh cleachtadh anois (a chleachtad because following a verbal noun, another verbal noun doesn't come into the genitive (see Gramadach na Gaeilge section on genitives, under nouns).