To answer a question from your first post, you cannot use a locking clip with a booster seat.
Because boosters are not actually installed, they tend to move around quite a bit more than carseats. It's alarming at first, but quite normal.
As far as the fit, the booster seat needs to make an adult seat belt fit on a child. That means the lap portion of the seatbelt needs to be low on the hips, and the shoulder portion of the seatbelt needs to cross at the collarbone. In the pic you posted, the booster is doing both, so it's a good fit. :thumbsup:
I think some of the responses got confusing - height does matter with a HBB. The tops of his ears need to be below the top of the headrest portion of the booster, and the belt guide needs to be at or above his shoulder. If the belt guide is below the shoulder, it's not quite as big a deal as it is with a carseat, but it will almost always cause the seatbelt to be improperly positioned.
It looks like he'll be using this booster backless before too long, at which point you need to make sure the vehicle headrest is properly positioned, with the child's ears centered between the top and the bottom of the headrest.
As for returning the Monarch and getting a Monterey instead, the benefits of doing that would be that the Monterey HBB will provide head support longer than the Monarch (which shouldn't be an issue for you as your vehicle has adjustable headrests, yes?) and the Monterey HBB will provide more side-impact protection (does your vehicle have side-curtain airbags?) Another factor is comfort if your child falls asleep in the vehicle - the Monterey HBB will give him something to sleep against. If the sleeping, SIP, and head support aren't issues for you, then the Monarch should be fine.