A trick to the Boulevard....
The knobs to adjust the height of the harness on the side.... the actual hole in the seat should be even with his shoulder when adjusted correctly. You have to reach through the cover to feel it... adjust the harness/headrest so that it is about even with his shoulders.
Then lower it further... 1/2 an inch or an inch below his shoulders.
THEN tighten the harness as tight as you can get it which still won't be tight enough for you.
THEN raise the headrest using the knobs on the side and bring it back up to even with his shoulders. Raising the headrest will tighten the harness, therefor eliminating the tugging and pulling to tighten it with the adjuster strap.
You'll have to play with it a little bit, because you may have to lower the headrest more or less depending on your kid and how tight you can get the harness, ect. However, this works with my girls. And, I leave it tight I don't loosen it to take them out...then I just fight with it to buckle them back in.
Easier than making sure it is tight enough every single time because I tend to obsess a little about that.
Hope that helps!