How would you manage this contractor if you were me?

[Tareq Salaita]

Hello everyone, I wanted to summarize an issue I'm facing with my contractor and get feedback from the community here about how you would recommend I proceed. Here is a summary of the situation:

• In December, 2015 I purchased an old home that was not in living condition (3 bedroom/1 bathroom home)
• I signed a contract with a design-build contractor to upgrade the home into a 4 bed/ 4 bath -- contract included budget, details of work to be done and payment schedule. Project was supposed to be completed by July but we have been going through problems with permitting, unexpected things coming up..
• I have 2 more payments remaining on the contract-- my contractor asked if he can get them in advance to finish the project 'in the next few weeks'. I don't feel comfortable in doing this as he has not finished the kitchen at this stage, tiling, bathrooms, etc.. 
• To make things more complicated, I already have a seller who wants to put the home under contract and make 'additions to the home'. I don't trust my contractor in completing the original work on budget and don't want to get the new seller involved with my contractor as not to distract him further

My question is: Should i continue making payments although my contractor is behind schedule ( he has already submitted 2 change orders which I paid for)?  Should I advance him the last 2 payments? How do i know if the change orders he is sending me are legitimate ? Does it make sense to bring in a second contractor to check on the work and give me a 'sanity check' of where the project really is?

Thanks!

Tareq