What breed are they? Some others on here have different opinions, but if they're a traditional breed and are raised outdoors in a small group, I wouldn't be worried about keeping them to past six months. I've often kept entire Tamworth boars to seven months without detecting boar taint, nor have I had complaints from customers.
Commercial breeds and cross breeds will mature earlier, however. Check the withdrawal time on the medication and just see how he goes, but I wouldn't fret too much about hanging onto him a bit longer. Your first duty is to make sure he's okay.