I don't think that he will be able to prevent the baby from learning to walk, don't worry!
But I do agree that it's important to physically move the 3yo away or hold him (gently but firmly so he can't wriggle out) far enough away to give the baby space, rather than just talking to him. Or you can do something like time out in a playpen, but I don't think it necessarily needs to be a huge thing. Just take him away and involve him in something else.
Or another thing to do is try to figure out what the 3yo is trying to do - is he wanting to help his brother walk? If so you can redirect into something helpful like suggesting that he fetches a walker for the baby (like the VTech one or a weighted wooden one which won't tip up). Or show 3yo a game where you both stand/kneel at one end of the sofa and encourage baby to come to you while holding onto the sofa, and then do different "challenges". Or get 3yo to build some towers for the baby to knock down.
Is he wanting the baby to get out of the way? You could teach him "excuse me" and show him how to walk around carefully.
Is he wanting baby to sit down because that's how he's played with him so far? You could encourage him to set up a game and then wait for the baby to come over and play. My middle one has always struggled with waiting and one thing which has helped is to get some sand timers from amazon and I pick a time which I think is a reasonable length of time to wait (usually just 1 or 3 minutes for that kind of thing, maybe 10 minutes if he was waiting for the baby to finish with a toy) - it just helps them visualise the time. But he has needed me to be very physically present to help him wait which might have meant holding him back before he just ran to get whatever it was he wanted. While you are waiting, you could talk about the stages of learning to walk. Maybe also look for some TV programmes or books about a younger sibling learning to walk - I expect there are lots with different characters where there is a sibling relationship. For example, Daniel Tiger is usually good for sibling stuff, especially since Baby Margaret is that much younger, Peppa Pig is also popular in our house and Bluey - though the younger siblings in those programmes can already walk, so you might need to search for one where it's more retrospective. Wonderblocks on BBC is a new favourite of ours as well because it has a lot of "Stop, wait, think" and talking about things happening in the right order, otherwise they don't work. Alfie and Annie Rose I think there is a book where Annie Rose is learning to walk as well. I know there's one where she learns to talk and says Alfie's friend's name before Alfie's and he is upset about it, but it doesn't take long before she starts talking more.
Does the 3yo go to nursery/preschool? Baby should get some time and space on their own then. Something which I wish I'd realised is the importance of carving out space for each child when they are close in age like this, so e.g. at the weekends when your partner is home, one of you should take one out for an outing on their own, then the next weekend you swap, so each child gets one-on-one time, both at home and out and about, with each parent. And make sure to identify potential same-age playmates and invite them over to play so that they have other children to play with rather than only ever playing with each other, and have some experience of not always being the centre of each others' attention. I thought it was great my two youngest are best friends, and I do think it's nice they have that bond, but I hadn't realised how dependent they are on each other until recently (they are 3 and 6 now), and I was reading recently that when they only ever play with each other, it can make it more difficult for them to learn social lessons about how if you're mean to people or hurt them, even accidentally, they might not want to play with you in the future. Siblings don't have that option, they are always still there and will play with you anyway.
Lockdowns prevented us having many playdates when they were little, and we didn't get into the habit of it but looking to change that now.