I found out my boyfriend likes it and is a “feeder” and secretly fed me in the past.
How did you find out? Did this lead to you confronting him?
I’m exploring the idea of doing small intentful gains for his pleasure but I am worried it could get carried away and that would make me feel uncomfortable.
You need to establish boundaries; asking this question in a public forum for advice is a good start. Talking to him and asking about his expectations should be your next step. Negotiating kink, is, by definition, a negotiation. Find out what each of you want. Figure out what you are and are not willing to give each other. Meet in the middle, or as far as you each are comfortable with; if that gap is too large, the kink should not proceed.
What are good limits to set and has anyone had a successful submissive healthy relationship and if so how did you manage? Examples wood be helpful.
Ask yourself what "too far" would be like for you. Do you want to maintain or attain a certain lower weight than what he might desire? Do you want to be able to remain physically fit? Are you worried about outside judgment? Only you can answer these questions for yourself. Once you know them, communicate them to your partner.
How do you limit non-consensual feeding? I don’t want surprises in my food again.
Telling him that you do not want any more non-consensual feeding is a good start, and explain why.
However, the larger concern I have is that you worry about surprises in your food. If that is the starting point for this kink, I'm not sure you should proceed with the kink at all until he can prove he's wiling to respect your boundaries.
It would be easier to give advice if we knew how exactly he surprised you in the first place, but if it was something in the vein of adding real sugar to your food and lying that it was artificial sweetener... well, that's a betrayal of trust that I wouldn't forgive quickly. Don't accept food from him at all, if that's what it takes.
Frankly, I'm close to suggesting you end the relationship because that kind of deceit disgusts me, but I shouldn't jump to conclusions. Trust your instincts. If it feels like abuse, call it out. If he dismisses or reacts negatively to you asserting boundaries, dump him as soon as it's safe to do so.
How did you measure or know she/he gained weight if the boundaries are set to only have small gains? Is it pointless to start if I don’t want big gains and will he just want more in the end?
There are people in this community who are feedees and eat for their partners but intentionally lose weight or maintain a low/healthy weight. Similarly, there are people who don't mind feedees who are thin and would even prefer occasional bouts of stuffing over secret feedings to induce a long-term gain. I have a guess as to which your boyfriend is.
This is a matter of personal taste and it will depend on what he wants and how much he can demonstrate respect for your boundaries. If he seems like he will push those boundaries, don't bother with indulging his kink at all.