A properly done Manual-J would more than likely just cement the case for a smallest-in-class boiler. If the sole purpose of the exercise is to size the boiler, it's a waste of time. Show him the napkin math on the fuel use.
But Manual-J can be useful for figuring out where envelope upgrades would add to comfort & efficiency too, if they're willing to run the numbers on the building as-is, and again with tighter air-leakage plus some insulation upgrades.
Air leakage is almost always worth reducing (and unless they're out there with a blower door their numbers will just be a boilerplate WAG), and fixing all the gaps in the insulation is usually worth it too, eg:
The biggest thermal gap in most pre-Y2K homes (as well as the biggest air leakage) is at the foundation walls, foundation sills, & band joists. A poured concrete foundation has an R-value of about R1, the band joists are about R2, and even if you let the basement run a 55F when it's 15F outside, that's a 40F delta on an R1/R2 surface area, losing 20-40BTU/hr per square foot. In an 1800' a 2-story house with a 900' footprint there's 120-150' of perimeter, and the band-joist alone is on the order of 1500-2000 BTU/hr of heat loss, and if you have ~18" of exposed foundation that'll be worth over 5000 BTU/hr when it's that cold out, making it well into double-digits as a percentage of the total heat loss, and that's with a less-than-cozy basement temp(!).
Putting 1.5" of fire-rated rigid Thermax on the basement walls & band joist with edges taped & sealed reduces that part of the load by about 90%, and the basement would than run in the mid-to high 60s F, increasing the surface temp of the floor on the first-floor by a few degrees, with a noticeable improvement on barefoot comfort, and reducing the summertime relative humidity to boot.
Spray foam does a better job of air sealing the band joist & foundation sill, and goes in quicker than cutting & cobbling rigid foam, but at the very least spray foam would be used to seal the crack at between the sill & foundation, the sill & band joist & subfloor, as well as any cut rigid foam at the band joist & sill.
Air sealing floor/ceiling between the attic and conditioned space is the other biggie from an air infiltration point of view. Fixing the air leaks at at both the bottom & top of the house mitigates the stack effect pressures far more than any leakage that happens in-between, with a direct reduction of both heating and (primarily latent) cooling loads. Even a blower door test number doesn't tell you WHERE the air leaks are, and models that use a blower door number to come up with an infiltration rate are by nature extremely imprecise, if better than a WAG.