When you go into Edit Town there is a button which looks like the Symbol for CAS, it's a big square with the outline of Sims on it, that takes you to CAS, make your Sim, click the tick, then he will appear in what we call The Library, which stores all your Households, click on him then go to the House where your Sim lives, click on the tab and an option to merge the households together should appear, click accept. Then return to game and the Boyfriend should be there.