Have a Weil Mclain CGS Gold boiler. There is no domestic hot water unless I turn on the CH. This was no problem during the winter as the CH was always on, however, with warm weather the CH is usually off. Should I assume a thermostat issue between the boiler and the tank?

Sure...

Could be the aquastat (thermostat) or the DHW relay (if present). One easy way to test the low-voltage aquastat is to jump the control (i.e. spin the 2 wires going to the aquastat together). If the boiler fires, you found the problem. Testing the relay is harder to discuss without knowing the exact relay you have.

Mark