Dear customer, thank you for your interest. This product is a house slipper and is generally considered to be a regular fit (standard mold). Although we cannot make a clear size recommendation, it may be appropriate to choose the size you use daily for regular fit products. If your boyfriend wears 43.5, we kindly ask you to examine the size 43 and 44 options in detail with the size comments. Especially in product reviews, the foot sizes and experiences of other users can give you an idea. We recommend that you review user comments and size chart/characteristics to choose the most comfortable size for you.