I wouldn't bother with a bottle for a 15 month old. Just put the EBM in the same kind of cup that he uses to drink water. You may even find it helps in learning to drink water, if he can start off with EBM in the cup. I remember my oldest girl at 17 months old calling EBM in a cup "special yummy" (I was pumping off the excess from my oversupply with a newborn).
Also, if you can't pump much EBM for the 15 month old, don't stress about it. He may be happy to have an early feed before you go out and then let daddy settle him to sleep without EBM. Good practice anyway, daddy might need to settle him to sleep more often after the new baby arrives as you'll be a busy mum

Pumping is something that is easier for most women when they have younger babies and stronger supply, and letting down for a pump or hand expressing takes practice. You may find that even if you can't get much now, you get heaps when the new baby arrives.
With the newborn, I think it's really important to let your supply settle down before you start expressing, so you don't end up with an oversupply problem. About 6-8 weeks is normal for most women's supply to settle down. When you do start expressing, you don't have to use a bottle if you're concerned about nipple confusion - some babies refuse the breast after trying a bottle, other babies (like mine) would rather starve to death than accept a bottle. You can feed a very young baby with an open cup - they just lap milk over the rim of the cup. Older babies (around six months) can start with a straw or sippy cup.
The
Australian Breastfeeding Association have downloads on expressing and cup feeding, and their counsellors are great.