Hi and welcome to the forum
It's great that you are so invested in learning all about the care and willing to offer him a home
A vet visit would definitely be a great idea (hopefully there is an Avian vet that you can access?)
Do you have any pictures you can share?
As for diet, they are stubborn little buggers but seed only is definitely going to cause damage down the track. Buy some crumble from the vet or pet store and introduce it through the seeds to start with - same goes for veg & fruit - sprinkle seeds through it. You will need to get creative and perhaps try making crumble & mashed veg balls and cook them in the oven. Also present the veg in different forms such as shredded, mashed, stickers etc. and figure out what he likes.
Remember to properly wash his water and food dishes every day and keep everything fresh - particularly the water dish - often the mistake is made of just emptying it and refilling it. Over the course of the day water builds up bacteria and a slim not visible to the eye can grow around the sides of the dish which can be harmful.
Another very important thing to remember is that any toys,swings or perches that have rope or any other fabric material is da goujons to the bird and should not be used - birds have a habit of chewing everything and the small fibres are extremely harmful if digested. Stick to wood and plastic. Steer clear of metal also because rust can cause heavy metal poisoning.
Hopefully I haven't bombarded you with too much info!!