Howdy Hoseppy and welcome to Parallax Bill's Yugoslav Mauser forum!

Seekers of history. You're our kind of folks because that's what we do here.

Your rifle was manufactured in Germany for the war and is a standard length Mauser action.  It may have been taken by the Yugoslavs during the war or received as reparations after.  In keeping with Yugoslav refurbishing practice, original German markings were scrubbed off, any repairs needed were made, and the weapon was refinished, which the Yugoslavs typically did a good job of.  You might find the occasional waffenamt on some parts.

Preduzece 44 was the main Yugoslav arms works in Kragujevac in present day Serbia. It is still doing business making firearms and other related gear as Zastava Arms.  This is also the place where the "Yugo" car was made if you recall those.  I assure you that was a different part of the operation and their arms are excellent.
The "Mod.98 on the side would be either the original German marking or possibly it was ground and restruck by the Yugoslavs though, in this case, not likely.  From 1950 on an additional stamp "/48" was added behind the Mod.98 and scrubbing and restriking the numbers was more common.  Variations in stamp fonts will tell the tale.  In some instances, the Mod.98 (or M.98) can give some clue of the rifle's origins. For example the Gothic font "Mod. 98" indicates manufacture at Erfurt through 1941 I think.  I am not so good on German Mausers.

The serial numbers are Yugoslav assigned/applied numbers.  Note the absence of the bolt take down disc? That is a Yugoslav made replacement stock.  It's hard to tell from your photo but it may be elm.  In case you've heard the tale, be advised there is no such thing as teak stocks on Yugoslav Mausers.  That was/is simply blatantly false advertising.

You didn't post every mark but I would hazard to guess you will see a circled "BK." BK is the Cyrillic alphabets version of our script "VK" and stands for Vojna Kontrola or "Military Control."  If there's a #3 on the stock cross bolt, that is a Cyrillic alphabet "Z" an inspectors stamp.  The myriad other small stamps are not known due to records having been lost or destroyed.

There. Does that help you out?

By the way, just in case you don't know, any surplus military ammo will be corrosive. Clean asap after shooting and preclean with water to dissolve corrosive salts left from the primers, the source of the corrosive elements.  Make sure to wipe the bolt face too.  You can use gun solvent for corrosive residue but it really isn't necessary if you just do as I suggested.

American brand 8mm ammo will be milder than full power military or Modern European ammos and may be more comfortable for a 14 old to shoot and of course is more forgiving about cleaning if that is a concern.

If you have any more questions, feel free to ask away.

May you and your son enjoy many a safe and pleasurable shooting session.  You should find these quite accurate though don't be suprised if the shoot high at 100m/yd.  By all means use a 6 o;clock hold.

Again, welcome to the forum.