My holland lop also had a growth spurt in the first 6-8 weeks after I got him at 10 weeks, but he’s slowed since then. He’s 6 months now and was 3.5 lbs when I weighed him a few weeks ago. So while I can’t answer whether he is indeed a holland, his growth spurt seems on target with his age.
As far as pellets go, I don’t think it would be harmful for you to continue giving him unlimited pellets, at least for a couple of weeks. But if you cut them back a little, I’m sure it also wouldn’t do any harm, though I would stick with the Young Rabbit Essentials and not adult feed. Do you have any idea of approximately how much you’re giving him right now? I started by cutting Hugo’s pellets in half over the course of a few weeks until he reached 1/4 cup, which is what he’s on now (I may lower him to an 1/8 cup if he doesn’t grow much more). That was when he was around 4.5/5 months, and he just got fully transitioned to adult feed within the last week. Since I’ve heard that smaller rabbits stop growing sooner than bigger ones, I felt comfortable transitioning him to adult feed a little sooner than six months.
Have you heard of false dwarves? If he does get bigger than 4 lbs, perhaps that is a possibility. Igloo is absolutely adorable, by the way!