I'm going to assume this is a $7,000 per month gig. Since the employer offers no retirement plan, there was no deferral made there. So only the business earnings matter.
Again, assuming August - Dec - 5 months X $7,000 per month. $35,000 this year in SE income. So husband, a proprieter, can do the max:
Employee side: $18K unless over 50
Employer side: 20% of (SE income less 1/2 self employment tax)
$35,000 * .9235 = $32,322.50 - adjust your income down per IRS rules
$32,322.50 * .153 = $4,945.34 - this is self employment tax
$4,945.34 / 2 = $2,472.67 - one half of self employment tax
$35,000 - $2,472.67 = $32,527.33 - SE income less 1/2 self employment tax
$32,527.33 * .2 = $6,505.46 - Maximum Employer side contribution
Total max SoloK contribution on $35K of net self-employment income = $24,505 (rounded to nearest dollar)
Send at least $18K in before year end, then you can figure the exact amount and add the rest before April 15th. What I personally do is keep a running total of $18K + 18.5% of my income as I'm paid a variable amount each month - send that in as money is available to do so. Then I figure the exact amount and send a small amount to get right up to the limit in January.
Incidentally, if you meant $7,000 until the end of the year, then another limit comes into play - 100% of SE income less 1/2 SE tax. That would be $6,505. This limit is not often discussed, because most people contemplating a soloK have enough income that it doesn't matter.