Hey Jennifer. Really sorry about your father's illness. Are you familiar with transfer boards for getting in and out of the shower/tub? They extend over the edge of the tub so you can sit down outside the tub, then swing your legs over the edge, then slide your body until you're inside the tub. On some of them you scoot your butt on the transfer seat. Others have a rotating and sliding seat that rides on a track.
In the short term, elevating the chair or sofa on spacers placed under the legs will make it a lot easier for him to get up and down. That, like every other accommodation is going to be temporary. But with ALS that's typical of what you have to deal with - like trying to hit a moving target. You're probably aware of lift-assist chairs that give you a boost when you're standing up. They too, are going to help for a while.
If there is someone to assist him, then there are techniques to assist him. You basically get in front of him, get a secure hold under his arms, then in coordination with his effort get him on his feet and immediately pivot 90 degrees and lower him into his waiting wheelchair. It's all one smooth movement. You can also get transfer belts that give you a secure handhold on him.
Have you made contact with a local ALS association? If not, I'd highly recommend you do that as soon as possible. They are very helpful and maintain loan closets of useful equipment, such as shower transfer seats, transfer belts, power wheelchairs, hospital beds, and most anything else you'll eventually find you need. They can also show you how to use it.
Your dad is certainly fortunate to have such a loving daughter in his corner. I pray peace and grace to you both.