Yup - consistency is the thing.
I trained my little guy at 2 yrs, 10 mos. I had been talking it up to him about being clean, not wearing diapers, being comfy not scratchy, and wearing undies like mommy and daddy. (any known bigger kid will help too). And we read new books and got a potty video.
I trained him first for doo-doo while still in pull-ups. He had a pretty good schedule so I could predict, and I bribed him with shopping for a new toy....which had been denied for about a month. He was so excited to put doo-doo in the potty so he could get a toy (I took him to the dollar store!). Then I went down to yummy things and pre-bought toys wrapped up , then finally just stickers as the days went by.
After that happy success it was almost fun for wee-ing!
We picked a weekend when hubby was home and nothing was planned but staying home, and I told the little guy about the big day we'd start, like it was a party to look forward to. And, daddy was very involved that first day. We did it like the pre-school and put a stool in front of the potty to stand up to pee like daddy.
Son wore just undies, they drank together and went to the potty every 30 min. at first. They have to learn the signals from their body...they don't know at first...talk to them about what it feels like and where (lower tummy). You can confine him the fist day to an easily cleanable space, and let him play in the back yard. There will be cleanups the first day, but I had maybe one the second day! Stay with it - they earn stars and maybe special stickers and then for every row completed on a chart, they get a little toy or something they want. The trick is to make them WANT to go potty to get something they want. My little guy TRIED to go wee every 15 minutes just so he could earn the toy!!
After a week, he started to learn when he had to go.
We kept him in pull up for beds, going to the potty just before sleep, and first thing in the morning.
We learned to carry pullups and a pair of shorts with us in a little plastic bag when out and about.
Since you've been back and forth, it might be hard for him to know you're serious this time, but make it a big deal and be prepared to bribe and follow through. We also had him (help)wash his undies when he missed...but not as a punishment.
I hope this helps. You can help him look forward to this! Your younger son is probably too young to do it independently. There are ideas about training babies, but I personally think it's more about training the mom when to anticipate their needs - too much effort...FOR ME.
Good luck!