I was thinking loaning him the 15k and take the house as collateral.

Basic leverage methods would be to Sale and pay taxes, Sale and do a 1031, or collateralize this asset to buy another asset. 

We gave the buyer a 0% hard money loan collateralized by the property in under 48 hrs.

We have been able to significantly force appreciation due to renovations and then use the renovated properties as collateral instead of using cash for our down payments.

You could potentially do a cross collateral loan and get a way with the 15% down using private money. 

You can do a cross collateral hard money loan to cover the remaining DP if you have equity. 

Permits are usually cleared in about 4 to 6 months and are free of charge.The developer usually charges a 5% deposit to the buyers; once 70% of the lots are reserved the money is held in escrow but used as collateral / downpayment by the bank I hardly see any interest in running this type of development if it has to be financed all cash; any leveraged conventional investment in a rental or rehab would yield a much better cash on cash return with less risk 

This will be trying to find the needle in the haystack.If you had them under one LLC you may have had a better chance but lenders today are very reluctant to cross collateralize multiple LLC's.

@Lilly FangNo experience - what are they offering as collateral and projected returns?

This will show the bank how you intend to fund the mortgage with rental revenue.Be prepared to provide collateral and establish the needed down payment for the loan.