Your son is probably going through a totally normal phase, but I understand the stress it causes. At 18 months, a lot of kids still prefer breast milk because it's familiar and easy, so it's not unusual. The thing is, at that age he already needs nutrients that breast milk alone doesn't provide - iron, zinc, varied proteins. The good news is it's not about "forcing" him in the sense of making him eat; it's more about offering food without pressure and letting him explore at his own pace.
What a lot of parents don't mention is that sometimes the rejection isn't because he doesn't want to eat, but because the texture or the way the food is presented intimidates him. Try this: let him eat with his hands, no spoons. Give him small pieces he can grab (banana, toast, soft fruit chunks or shredded chicken). That way he feels like he has control and it's less threatening than someone shoving a spoonful in his mouth. A lot of times when you let them choose, it's surprising what they end up eating.
In the meantime, if he's gaining weight, has energy, and the pediatrician says he's doing well, breathe easy. Don't take away the breast overnight - that would just make him cling to it even more. Just offer food at family meals without making a big deal out of it, eat yourself so he sees you doing it, and leave it at that. Some kids take a bit longer to get into solids, but eventually they do. Pressure is the enemy here.