I hope you didnt pay much for it, thats an old board....  2014 ?

https://cpu.userbenchmark.com/Compare/AMD-Athlon-II-X4-860K-vs-AMD-A10-7850K-APU-2014-DKa/3265vs2937#:~:text=The%20AMD%20Athlon%20X4%20860K,APUs%20including%20the%20A10%2D7850K.&text=Comparing%20the%20CPU%20performance%20of,two%20processors%20perform%20almost%20identically.

We need to know what your son uses PC for and other components to recommend something sensible.  You'll probably want to go second hand and cheap otherwise you could buy a brand new cheap mobo\cpu combo that outperforms any of the CPUs compatible with that mobo. 

Basically there are ~4 generations for these FM2 platforms "Godavari" or "Kaveri" are the best bet 
https://www.asrock.com/mb/AMD/FM2A68M-HD+/#CPU

Godavari are newer so if you can get those they will be a bit better (and newer) .. try and avoid the older ones.

So basically your looking for any of the "A10-xxxx" or "A8-xxxx" .. but more cores and higher clocks are your best bet if you plan to do anything other than surfing the net.

If your just surfing the net and doing homework then I see some on trademe for $20~$30
A4-4020, A4-3400 and A4-5300 .. etc
This A6-3600 might be best "value" - https://www.trademe.co.nz/computers/components/cpus/amd/listing-2718499271.htm
Or this A8-7600 ..  https://www.trademe.co.nz/computers/components/cpus/amd/listing-2725799212.htm

As mentioned though, this was pretty poor platform and something like the old Intel I5-2400 on a H61 board would provide a better experience.
I have seen these combos for $50~$70. Saw a whole PC with I5-3570, RAM etc go for just $80 on trademe last week.

Good Luck.

basiclly he plays what all the kids these days play, fortnite and roblox, got a good gpu,

his board died in such a way id never seen before. just turned off , and no would not turn back on. had power to it, but no power to the switch , tried force power up by bridging the power pins - nothing .

What GPU ? If its a lot newer than the CPU, will hit bottleneck.