If you buy the house jointly, in a trust, you can avoid a lot of needless complication. 

In order to prepare for our joint venture I am doing a live-in-flip in Rhode Island were my current job is.

I have better credit and income, however, so we would like to take the HELOC out jointly to get a better rate. 

Have you considered doing a joint venture partnership?

@Aaron Deane Many of the answers you are looking for can be found here:How to Effectively Conduct Joint Venture Agreements as a Real Estate Investor596231-how-do-i-properly-construct-a-purchase-with-a-partnership591376-create-an-llc-for-first-partnership-best-way-to-do-so526244-taking-on-partner-s-and-limiting-our-liabilitesGood luck!

Keep totally separate accounts, merge everything, or make a stand-alone new joint account? 

Can we simply add him onto my account as joint holder in order for him to have proof of funds to close A.S.A.P and not run the risk of loosing the property?

So, in this case, would having the son added on the lease give the dad the same flexibility to break the lease when the deployment happens because the lease is jointly in their names?

You might want to do a Joint Venture with someone that provides the financing.

If coming up with the down payment is an issue, you could find a joint venture partner - someone who has the cash and is willing to partner with you on the project.