Welcome to the Forum. Brilliant news that you have some hogs visiting there.
I can understand your concern and you’re right that some hoglets need help over-wintering, but it’s really too soon to be worrying about them being underweight yet. Hoglets tend to put on weight surprisingly fast and it’s likely that if that hoglet is out and about on it’s own already, it will have plenty of time to put on enough weight for hibernation. It’s actually very stressful for hogs to be in captivity, so it’s best avoided. As long as you keep feeding the hoglet it has a good chance of being able to stay in the wild, which is the best thing for any wild hog.
The hogs probably will gradually disappear off to hibernate. In the past I have found that it’s normally the adult males who go first, then the adult females and lastly the hoglets. Although there are always exceptions. Occasionally a hog will decide not to hibernate and if that happens, just continue offering food and water. I have had hogs not hibernating here before and they continued to visit all winter even in freezing temperatures and one year when there was snow.