You could indicate availability using the calendar. Keep max units at 100% and reduce the available hours on his calendar from 8/day to 4/day during that time period.
Ah, more information. That helps. Basically the KISS approach is to set all tasks to maximum assigned units and then use predesessors or other techniques to level the tasks. That would schedule Task-B for 1.5 days and then Task-A for the next 7 days. The net result gives you the end date from both tasks. Hope that helps.