To give him comfort, you can suggest opening a joint bank account.

For now, put all your down payment funds in a joint account (both names on the account, not just one that you "consider" to be a joint account), and plan on buying each rental property in just one person's name.

He has joint custody of 2 children from previous marriage and I believe he gets to claim one of them on his taxes.

I know plenty of investors that do just that  and enter  joint Partnerships  or who strictly do wholesaling  which is someone who finds then brings  off-market deals  to investors  and either reassigns the the house that's under contract to the investor for a fee or enters a partnership and turn it into a flip.

I am considering joint ventures in Florida market, specifically multifamily deals. 

I figured 20k would be a very healthy budget for rehab.

I am early in my real estate investing career with a few properties under my belt and gaining some steam as a working partner in Joint Ventures.

This is called "joint and several liability" where every individual is 100% responsible for the terms.2.

Take your low interest HELOC loan and invest it in high interest secure real estate investments as a joint venture money partner or as a lender secured by a mortgage.

I would either go to couples therapy to repair your relationship or dump your partner and have him buy you out of your share of the joInt property or you buy him out of his share and take over.