Have you taken him to a rabbit savvy vet? They will tell you the proper diet for his age

As a holland lop, especially if he’s a true dwarf, he’s not going to get past 3 pounds. He sounds about right for his age. My boys were both 2 pounds at 12 weeks, but they are false dwarf holland lops, so they’re both 5 pounds.

As for diet. Are you feeding him timothy pellets or alfalfa pellets? Are you feeding him timothy or alfalfa hay? I’d also cut back on the oats. Oats are an okay treat for a baby bun, but I wouldn’t feed them to him all them time as a standard in his diet. If his pellets are timothy based and he has timothy hay, it’s a good idea to mix some alfalfa hay in with his timothy hay to encourage proper growth. Bunnies under 6 months should be fed alfalfa pellets as this will help them with proper growth and nutrition for their age.

He probably isn’t showing interest in his pellets because the oats are mixed in. Try feeding oats at a separate time than the pellets. The mixture of oats and pellets may seem like a good idea, but it leads to problems when the bun eats the oats and leaves the pellets (which contain most of the essential nutrients) to get stale.
Opinions on oats range from not feeding them at all to feeding them as a regular addition to the bun’s diet. I have fed oats regularly (in small amounts, like a tbsp or so per day) to my rabbits for years, including babies, with no ill effect. But if the oats are causing him to refuse pellets outright, I agree that you should decrease or stop feeding the oats until he grows a little more.

Because he is on timothy for both, mix in some alfalfa hay with his timothy hay just so he gets extra bulk, or buy some alfalfa based pellets. it’s easier to transition from alfalfa pellets to timothy pellets at 6 months instead of alfalfa hay to timothy hay, so I’d suggest transitioning him to a young bunny pellet like Oxbow Essentials Young Rabbit food. It gives him the extra protein and fats he’ll need to grow healthy.