Son struggling with my partner staying over

I have been in a relationship for 2.5 years. I have children from a previous marriage and am a sole parent.  My partner has recently starting staying over at mine 2 nights per week for the first time.  My 9 year old son is autistic. Both my children have known my partner for a while and they seem to get on well with him, but my son is having big reactions to him staying overnight. He will not settle for bedtime and will get angry and keep waking his little sister up. He will wake very early these nights and come into us eg at 5.30am. If my partner is not there he and his sister don’t wake me until 7.30am (my survival being a lone parent) — usually my son prefers to stay in bed reading/doing crochet/iPad until I come into him. He is also shouting at me a lot on these occasions “Stupid idiot Mummy” etc.  My partner is very kind and patient with him. Any ideas on how to help my son with these changes? I understand he may fear he will lose me etc so I keep reassuring him I am here for him.