If your son is 15 months and you have been breastfeeding this long then most definetly with some scheduled pumping sessions you two will be just fine. The hardest part will be if your son can go a week without nursing. Make sure now he takes bm, formula, or regular milk before hand. If you haven't already try him with a nuby sippy cup. The large spout makes it ideal for nursing babes.
My girlfriend had to wean her son at 14 months because she was supposed to take seizure medication. She had a horrible experience and found out she didn't have to take the medicine after all. She hadn't nursed in almost 6 weeks and with the help of some mothers milk tea and fenugreek she relactated and went on to nurse her son for a few more months.
I'm glad you are saying you want to continue a bit longer. I nursed my son for 21 months and it was so sad how quickly we dropped nursing and after a month he even forgot how to latch. I still have tiny amounts of milk left and after a bath together his baby instincts kick in and he will smell my breasts like it triggers memories. The nursing time really goes by all to fast, make the most of it. Good Luck!