there are a lot of ways to do it, i think i've used most of them at one point
If he's spitting it out right away you can use your hands to block his muzzle closed around it, some just fight harder, some get the idea rather quickly. You'll have to play it out.
for a dog that doesn't want to hold it, i use the food bowl and a pvc or wooden dowel. I feed raw, but for this time period I'll cut up some natural balance since that's what we use for tracking and put it in their bowl.
They think it's feeding time and I set it on the counter and have them sit in front of me. If I have to place the dowel in their mouth, when their teeth touch it, i mark and reward with a piece or two. We'll do this over and over and they pretty quickly get the idea that mouth on the pvc or wood gets food. When they offer the behavior willingly I mark and give them the whole bowl and quit.
Next day same thing, rather quickly, usually just a session or two and they're grabbing the dowel as soon as I pull it out without any direction. Then I start to ask for more. They have to take it and hold it. Usually I have to hold my hand over their muzzle for a brief second to give them the idea. They don't have to hold for a long period, but more than just a touch and release. When we've used about half the food doing these reps over and over, on a good one, i'll mark again and put the whole bowl down.
at some point depending on the dog, after they have a decent idea of what i'm asking for, if they continuously drop it, refuse to take it, refuse to hold, I put them in a crate, put the bowl down outside the crate so they see it and leave for at least 20 minutes then come back and try again. This is usually a week or two in.
Whether they're dropping it out of anticipation for reward or because they dislike holding it, it clears up really quickly when they realize they get nothing. and their opportunity to get their reward is limited, they tend not to want to waste those chances
they have to have somewhat of an understanding of what you want though, but if they'll "take it" I think that's an idea enough. At that point they'll accept the dumbell, i think it works very well.
Other points for the "hold", i don't use it as an opportunity to build drive, do that with a toy. It's low key, clear emotions from me for good and improper behavior, but not over the top, especially at first. Once the dog has a good understanding, my good and "NO" get more animated, but not excessively.
I don't ever mark if there's any chewing once I start building duration in the hold. There is always a pressure on the DB or dowel from me pulling so they have to put pressure down to "hold it" when I mark. That can be done by hands or putting strings in the end of the dowel and pulling those. Hold it, means clamp and hold, not just sit in the mouth.
When I get a nice hold then we walk around with it, chewing gets a verbal, ah and they have to hold longer, if they drop it, there aren't many chances at this point. Everything gets picked up and they go away, no chances for at least a half hour. By the same token, when they walk around with it and change some positions and keep holding it, I don't make them do it 20 times, they get rewarded with the food bowl when they've done what I've asked. So either it's getting picked up, or it's getting put down at this point, all your repetitions should have taken place long ago
once i have that, it progresses much like that video, they come to front and take it, the push me back with it in a hold. Then it's placed between me and the dog and they have to take it and bring and build distance, then we'll move to actually throwing, and retrieving and even that has some steps so the dog gets it and turns to immediately come back without taking wide circles like some do