Need urgent advice on rat ual maturity?

just bought 3 rats from a breeder but it turns out I have 2 female and 1 male. thinking of getting male rat nutered when old enough so he doesn't have to live by himself. worried about risks to male rat in operation. also not sure how old they are (aprox 8 weeks but not sure), was so excited to get them I forgot to ask and can't get hold of breeder. i know i need to seperate them but want to do it as late as poss so male not on his own (he is the nervous one of the 3). if i seperate them, get him nutered, will he be happy to live back with females or will they have been too long apart for them to get along again - i've been advised that male rat has to be 3 months old before op but have been told they can breed from 8-10 weeks old. can you tell rat age from weight? females are 132g and 111g, male is somewhere in between I think (wasn't keen on being weighed!). don't want to stress male out by taking him away from his sisters earlier than I have to but also don't really want babies as would like them all to live together so they get maximum playtime out of cage. can anyone advise me? Emma