My yearling was weaned early, at 3 months (serious health issues with his dam), I know it's not 5 weeks, but, they learn quickly what they're allowed to do and what they aren't allowed to do. Sam was turned out with a mare and a gelding. He was an outcast for the first few days, being forced to stay to the perimeter of their grazing circle, but other than a few charges, pinned ears, tail swishes, and a few kicks, he wasn't harmed. They're hardier than they look and don't break as easy as we think. He will be fine. He has a natural instinct to submit to older horses, you'll see him characteristicly mouthing to the gelding and he'll quickly learn what a tail swish or ears back means. Don't be afraid. You don't want to end up with a spoiled foal who doesn't act like or horse, or doesn't know how to act toward other horses. My 3 year old mare was by herself from the time of weaning until the day I got her last month, so a little over 2 1/2 years... She had no social skills at all, and while she's doing better, she still has some issues with typical horse language. Listen to your vet, your vet isn't going to give you bad advice.