How can I lower the gear ratio on my son's bike? I got a SH Commencal 24&quot; MTB for my son. He loves it - except on climbing steep slopes where he runs out of steam.
It comes with a 7-speed 14-28T freewheel (Shimano MF TZ21) + Acera derailleur, and a single 36 tooth chain ring.
I added a photo in case it helps.
What would you suggest for getting the lowest gear ratio -- lower ?
The cheaper, the better.
I have basic knowledge of bike mechanics (but I'd use this as an opportunity to learn) and basic tools (but then again, there's eBay).
If it's over my head, I'll ask a bike shop to do it for me, but it will add to the price and be less fun.
The specific model of rear derailleur you have is an Acera RD-M360 SGS, produced about 2009-2011, for 7 or 8 speed drivetrains. Shimano very helpfully provides archive specifications on the product info page.
Looking in the 2009 spec PDF we can see that the max size of the largest sprocket the derailleur will handle is 34. I believe 12-34 freewheels do exist, but you might have to hunt around to find one. That would be the cheapest way to lower gear ratios. (I'm guessing the crank on this bike does not have replaceable chainrings.)
Removing and installing freewheels is very straightforward, but requires a special freewheel removal tool to  get the old freewheel off, and there are many different types of tools for different freewheels. See this Park Tool Repair Help video for how to select the correct tool. Other than that you only a large adjustable wrench in addition to basic tools.
A larger range freewheel may require the chain to be a few links longer, which unfortunately means sizing and installing a new one. I installed an 8 speed rear derailleur on my son's single chainring bike recently and used the chain sizing directions for 1x drivetrains in this Shimano manual. This is working fine so far.