Working full time & breastfeeding

Okay I need ADVICE, tips, anything. I have an almost 4 month old and he's exclusively BF. I've been a stay at home mom since before he was born and now I'm starting a full time job next monday, I don't expect to not have to eventually supplement or my milk decreasing some but I just want to keep him on the breast as long as possible. I currently feed on demand, for as long as he wants and as often. I have a stash of frozen breastmilk that would probably only last a week and a half. What I'm wondering is, how do I maintain my supply and how will I make sure he still wants a boob after each work day? All tips or similar experiences would be appreciated! I just have a difficulty with the idea of stopping just yet, It's like I need that bonding time with him lol.