Oh very
for you and your piggie that has gone and the one who remains.
3 and a half is middle-aged, but he's potentially got another 2-3 years left so yes, it is tricky.
Your chap is probably a bit long in the tooth to neuter (though it's more about their fitness than years) but, boars do take well to a tiny boar piglet.
There will be a re-assessment period when the younger one hits puberty ( I had two brothers and we were in the same situation with GP1. We got tiny boar GP3 and gradually , he took charge. He did try it on with GP1, but the older one scolded him and walked away)
When the inevitible happened and we were left with lone boar GP3, we got him 2 girls to side-by-side with and had him neutered to bond when he was sterile,
Or you could get an older female (same age as him) to keep side by side to chat, but , they won't be able to live together properly . Rescues are full of singletons that are looking for rehoming.
If you get a piglet from Rescue they can boar date on neutral territory.
There's no guarentees that they'll get on, either male/male or male/female.
And of course, you're in a spiral of 'next pig' as you add to the list
Your piggie needs a little while to come to terms with it all, and lots of cuddles. (We put our lone boars in DD room at night in the cage when they were bereaved )