Suggest Ways To Determine The Paternity Of A Fetus

I had a one night stand, unprotected sex feb 20 at night so feb 21 he did not ejaculate inside me and did so after pulling out 5 min later. Feb 22 I started cramping and my period started feb 23 which ended 27th. I then had unprotected sex with a new guy I started to date and he did ejaculated in me on march 2, 6, 8, 11 12 and 13. I had an external ultrasound done today April 16 and they said I am 7 weeks and 3 days pregnant. Last week they only saw the yolk saved and fetal stem But no fetus. I know the baby is his and I must have ovulated on march 8 and conceived March 9th. The estimated date is Nov 30. This has cause a huge fight with me and my boyfriend. Does 7.3 mean the fetus is actually 5 weeks and 3 days old? I want to prove to him it s his. I know it is. Please help!