Actually yaar, it's quite safe but only if you are comfortable. Breast changes make them super sensitive and sometimes it can trigger minor contractions late in pregnancy, so better be careful.
Haha it is totally normal to wonder about this! Just don't let him go too wild because your boobs are already super sensitive right now. Keep it gentle.
Omg even I panicked about this during my 5th month thinking it will cause labor pain or something. Actually my doctor said mild stimulation is totally fine, unless you have some high risk issues or preterm labor risk. Relax, it's normal.