Assuming this is the only plan and the only eligible participants you can make a 25% of pay deductible employer contribution. Further assume PY=FY and pay based on PY.
For simplicity assume mom made 50K deferred $2K and got $2K match, son made $100K no deferral or match.
Eligible pay is $150K, 25% is $37.5K
Company has already contributed $2K employer match so it can make a $35.5K profit sharing contribution.
Since you said document has pro-rata allocation if company made max deductible 35.5/150 = 23.67% of pay.
Son would get 23.6K, mom would get $11.8K (within rounding).