I am sorry about your miscarriage, OP.
This is an interesting topic because I don't think there has been much (any?) scientific/medical research on how to avoid pregnancy with one partner while trying to get pregnant with another partner. With polyamory becoming more popular, this scenario may become more common.
If you have a poly-friend ob-gyn, it would be worth asking them. I am curious what birth control they'd suggest you use with your BF.
Condoms and spermicide? Diaphragm and spermicide? Plan B for emergencies would likely interfere with your fertility with your husband. Rhythm method probably isn't reliable enough. A vasectomy for your BF is too permanent

and the male birth control pill is still in development. What other methods am I forgetting? Female condom? (Does anyone use that?)
It does sound like condoms alone aren't good enough, even with the better type of condom that doesn't break as much.
Abstaining from vaginal intercourse with your BF until you get pregnant with your husband might be the best way to go. Other poly people I know have done that. The BF in that case was perfectly willing to take strict measures to refrain from risking pregnancy so that he didn't end up with a child he wasn't prepared to raise.
But your husband's insistence on the BF pulling out doesn't really make sense to me. Withdrawal is notoriously unreliable at preventing pregnancy...I guess with condoms it might work...but seems both risky and mechanically challenging. I would personally prefer to have no vaginal intercourse rather than worry about breaking condoms and timing the "pulling out" just right, etc. Sounds annoying.
And yes, insisting that your BF "pull out" does seem like it's an overreach in what your husband can control about you & your BF. Your husband absolutely DOES have a right to ask that you & BF use a backup birth control method or even that you not have vaginal intercourse at all while you're trying to get pregnant with him...but he doesn't get to decide WHAT method that is. If pulling out won't work for you & BF, it's your & BF's responsibility to find a method that will work, and communicate that to your husband so he can decide if that mitigates the risk enough for him. But it will come across as awkward, weird, and controlling if husband keeps having to ask you, "Did your BF remember to pull out in time?" It's too focused on the mechanics of what you & BF do with your bodies/genitals.
But, the price of autonomy for oneself is responsibility for oneself...so, it's up to you & BF to find a backup method of birth control.
And, I don't think your husband intends to be controlling...it sounds like he's trying to find a compromise so you can keep having vaginal intercourse with your boyfriend.