I would go with a bit longer, if he does not develop problems with his poop, and is eating other foods. I know folks wean foals at 3 months, foals do fine. However with him being a larger breed, he may need more time and the added nutrition.
I do not want fat foals, like to see a shadow of ribs showing when he turns. Fat is too much weight on young, soft bones that are just developing. He has a very long way to go before being mature, probably close to 7 years. Like obesity in kids, fat young horses are not what you want. This is DESPITE the advertising photos of fat horses in feed ads.
The TRIM young horse grows slower but steadily, not overloading any body parts. His genetics will tell the body when to stop, without tons of extra supplements and large daily helpings of grain and special "growth, young horse" feeds.
You do not mention if he has any companion horses. To me this equally important for a young horse, along with his milk replacer. Finding an old broodmare, a gelding who will keep the colt disciplined in good horse behaviour is REALLY critical to prevent foal becoming a bully and hard to handle. Older horse will play with him, can take the abuse of rough play that humans can not. Foal NEEDS to grow up with another horse, learn to be a NICE young horse with good example beside him. Many older horses, geldings, like young horses and take care of them in a herd setting. They give him a nip when needed, teach him herd manners, being a social nice guy. Gelding could be left with foal for years as he grows, unlike a mare if you want to keep the foal as a stallion colt.
We turned our youngsters out with the geldings after weaning. Those babies grow up to be the best horses to handle because they accept their herd place, accept discipline, since they never were top animal! They can be horses when out with the other equines, people need to be obeyed.
You might be able to lease a horse if you don't have others, for the companion animal. Lots of horses who need jobs available if you ask around. Lease might be just a "keep and feed him" deal, no real money charged. Don't get a kicker, that is a bad habit foal can learn or get hurt from. Biter is an acceptable discipline measure among horses. Mine had plenty of bite marks over the years, hair grows back and colts were better for being punished by other horses, for colts' bad acting.
Sorry to hear about the loss of your mare.