wifey6 - I'm struggling slightly to follow what's actually happening at the moment.
Are there two jobs that you were doing - one for 4 hours and another for 5 hours? And DH is only prepared to cover the one for 4 hours?
DH has Sundays to work on the garage/house, you say, so presumably the other 9 hours all fall on a Saturday?
My basic problem with what you're describing is that both of you seem to see things as solely the responsibility of one or the other - e.g. you see garage as something that is DH's responsibility, and DH sees DS as your responsibility apart from for 4 hours per week. Money seems to be another problematic area.
You both had DS together, so share the responsibility 50/50. Ditto, you both share the home, so deciding the priority your family attaches to finishing it is a 50/50 responsibility.
You need to sort this out asap for the sake of your long-term relationship and please, please don't bring more dc into this, as I suspect you will find that the 4 hours you currently get to yourself dwindles to nothing, as your DH doesn't sound like he could cope with 2 dcs.