Males grow faster if uncastrated, and if you have plenty of grass you should easily get him fat in four months or so.
Ewes usually don't get pregnant again till about 6 months after lambing if they are still feeding their lambs, so again you should have a bit of leeway.
Last year I grazed a texel ewe and her 2 lambs - one male and one female. The ram lamb stayed with her till she lambed again this year - almost a year to the day since her previous lambing. So obviously she wasn't mated any earlier from having her son with her.
So, if it was me I wouldn't castrate.