This app was mentioned in 9 comments, with an average of 2.00 upvotes
https://play.google.com/store/apps/details?id=com.oh1000.limitedvolume&hl=en_US It appears that you can do this on android, however android is not quite as user friendly as Apple products admittedly. Android allows for customization of the launcher that you will start apps from as well though. For example Big Launcher is often used for people who have vision problems or need a simpler layout with much less distractions. http://biglauncher.com/ Maybe if you have an android phone you can try some of this out. :)
Download this app on his S5: https://play.google.com/store/apps/details?id=com.oh1000.limitedvolume&hl=en&referrer=utm_source%3Dgoogle%26utm_medium%3Dorganic%26utm_term%3Dvolume+limiter+android&pcampaignid=APPU_1_0HAeWdjIKsKEjwOAu4nQDg
Use app lock so only you can change it, you can change whether or not it applies to the phone speaker, or when a phone call comes in. You can also let the user know the volume is being limited or not. Change Max Volume Limit to whatever you deem fit.
Whenever he tries raising the volume, it will automatically put it back down to the limit you defined.
Good luck!
Not sure if its a fix
The very bottom of the app app has downloaded app maybe it will or won't
Edit the app is volu.e limiter
You can sideload android apps. Find a volume limiting app (you might need to use a mouse to get to some settings). There are a few different options, just search volume limiter in the play store.
Here is one I found with a quick search:
I don't think you can do anything more than that on the stock software. I just found this app https://play.google.com/store/apps/details?id=com.oh1000.limitedvolume and it seems to work from the few minutes I've played with it. It just seems to block the volume slider from going past whatever you set the limit to. Not as elegant as a proper volume limiter, but I suppose you could try using this app or a similar one in conjunction with maxing the number of volume steps in SoundAssistant to get a little better control of your volume at the low end so you don't destroy your ears. You can also limit just the headset volume in the app so you don't impede the speakers. http://imgur.com/p2lTqdl
how about Volume Limiter (Limit Device Volume)? Link: https://play.google.com/store/apps/details?id=com.oh1000.limitedvolume