Well, let's assume that the bottles do contain botanicals used as despojos. Vodun (voodoo) as its traditionally called in the Dominican Republic) or Santeria (which is commonly found in Cuban and Puerto Rican communities but could have been introduced to Dominicans living in the U.S) is not viewed positively by most Dominicans. Unlike in Haiti, where vodun is viewed as both having positive (white) and negative (black) religious components, this syncretic religion (mixture of Yoruba, Bantu, and Catholic components combined into a new form of religious expression) is viewed by most Dominicans as being evil or a form of black magic. It is looked down upon as being something Haitians do but not real Dominicans. Herbal healers (curanderos) are fine as long as they don't utilize incantations to anyone but God, Jesus, and the Saints. Vodun potions are viewed as black magic and the invocations to the lwa or orishas would not be considered proper or acceptable by most religious Dominicans. Your boyfriend could be embarrassed to let on to you that he believes in the power of these potions to enhance his abilities. Alcohol certainly is an ingredient in many despojos. Many of the lwa or orishas favor rum as offerings.
Another point, a relationship without open communication and trust is not a solid one. Perhaps more effort should be made on your part to find out what, and for what purpose, is the liquid in the bottles used for. If he refuses to tell you, you can ask if these are despojos and that you won't think it's silly if they are or perhaps they are something illegal(?) and you are afraid of going to jail. You will be held responsible for what is in a house if raided by the police, even if you really are ignorant of the fact that the house contains these illegal items.