There is no risk from kissing, sucking nipples or mutual masturbation. His semen touching your penis isn't a risk unless it enters your urethra, and a drop isn't going to do anything.
So receiving unprotected oral sex puts you at risk for syphilis, genital herpes type 1, chlamydia, gonorrhea and NGU. You are not at risk for HIV from receiving (or giving) oral sex.
I’ll explain all the risks for everything, but most experts don't think a single act of oral sex warrants testing. If you have a regular partner you're concerned about infecting, you might want to test, but you probably don't need to test unless you get symptoms.
Syphilis isn't that common, and your partner would have had to have a sore in their mouth to transmit it. You wouldn't see symptoms of this for 10-90 days, but the average is 21 days, and you'd get a sore called a chancre. This doesn't cause burning, pain, discharge, etc. You can test for this at 6 weeks. If you get symptoms, but test negative at 6 weeks, test again at 90 days, and get to the doctor as soon as you see symptoms.
Herpes Simplex Type 1:
If you don't already have herpes type 1 (think oral sores, like cold sores but not canker sores), then you could get genital herpes type 1 from receiving oral sex. This can happen even if the person performing oral doesn't have a sore, but it's more likely if they do. The time from infection to symptoms is usually 2-12 days, but the average is 4 days. You can test for this now, and then again at 4 months to make sure you don't have it. If you test positive now, it's a pre-existing infection that you had before this encounter. About half the adult population has this, and 90% don’t know it. Ask for a type specific hsv1 IgG blood test. You don't need a type 2 test, and that test has some false positives on it, so avoid it if you can.
You'd see symptoms of gonorrhea at about 2-5 days, and this would usually be a discharge, burning, etc. Some people don't get symptoms. You can test for this as early as 3 days, but 5 days is better. You can have a urine test or a swab test.
Oral chlamydia isn't common at all, so getting chlamydia from receiving oral isn't likely, but has happened, so I mention it. The symptoms and time frame are similar to gonorrhea. A chlamydia test is usually run at the same time as gonorrhea, but make sure to ask for it.
NGU (nongonococcal urethritis urethritis, sometimes called NSU, for non-specific) is an infection in the urethra that is caused by anything other than gonorrhea . This can be caused by normal mouth bacteria entering the urethra, and other germs, like strep, adenovirus (usually causes upper respiratory infections like bronchitis), and the like. The symptoms and testing times are the same as gonorrhea and chlamydia.
Again, most experts don't think a single act warrants testing. If it helps your peace of mind to test, then test, but there's no need to panic.