Hi Stephane,
The first place I would start your research is the tutorial post here which has all the basic information, steps, and guidelines that can help you make the best possible choices.
Ultimate Dreams Eurotop Latex Mattress
With only 3" of latex … your boyfriend will sink into the support layers as well and I would tend to look for a mattress with higher density base foam (in the range of 2.0 lbs or higher). While this is a great quality and value mattress, would be a durable choice for most people, and has the advantage of being able to customize or exchange the comfort layer (and split them in king size) … I think I would look for either thicker comfort layers of latex or a higher density base foam to accommodate his greater weight.
16" LUCID® by Linenspa Natural Latex and Memory Foam Mattress Plush Four-Layer with Bamboo Charcoal Bed
You can see some comments about this mattress in post #2 here. This is a memory foam mattress (with some latex underneath the memory foam) and I would want to make sure that the memory foam was at least 5 lbs density or higher again to accommodate his greater weight.
With your boyfriend’s weight … I would put a particular emphasis on very durable materials. Post #3 here with more information and suggestions about heavier weights would also be worth reading.
Phoenix